diff --git a/frontend/src/app.css b/frontend/src/app.css
index 3a6fb71..1ea6b25 100644
--- a/frontend/src/app.css
+++ b/frontend/src/app.css
@@ -8,74 +8,100 @@
 
 :root {
 	--radius: 0.625rem;
-	--background: oklch(1 0 0);
-	--foreground: oklch(0.129 0.042 264.695);
-	--card: oklch(1 0 0);
-	--card-foreground: oklch(0.129 0.042 264.695);
-	--popover: oklch(1 0 0);
-	--popover-foreground: oklch(0.129 0.042 264.695);
-	--primary: oklch(0.208 0.042 265.755);
-	--primary-foreground: oklch(0.984 0.003 247.858);
-	--secondary: oklch(0.968 0.007 247.896);
-	--secondary-foreground: oklch(0.208 0.042 265.755);
-	--muted: oklch(0.968 0.007 247.896);
-	--muted-foreground: oklch(0.554 0.046 257.417);
-	--accent: oklch(0.968 0.007 247.896);
-	--accent-foreground: oklch(0.208 0.042 265.755);
-	--destructive: oklch(0.577 0.245 27.325);
-	--border: oklch(0.929 0.013 255.508);
-	--input: oklch(0.929 0.013 255.508);
-	--ring: oklch(0.704 0.04 256.788);
-	--chart-1: oklch(0.646 0.222 41.116);
-	--chart-2: oklch(0.6 0.118 184.704);
-	--chart-3: oklch(0.398 0.07 227.392);
-	--chart-4: oklch(0.828 0.189 84.429);
-	--chart-5: oklch(0.769 0.188 70.08);
-	--sidebar: oklch(0.984 0.003 247.858);
-	--sidebar-foreground: oklch(0.129 0.042 264.695);
-	--sidebar-primary: oklch(0.208 0.042 265.755);
-	--sidebar-primary-foreground: oklch(0.984 0.003 247.858);
-	--sidebar-accent: oklch(0.968 0.007 247.896);
-	--sidebar-accent-foreground: oklch(0.208 0.042 265.755);
-	--sidebar-border: oklch(0.929 0.013 255.508);
-	--sidebar-ring: oklch(0.704 0.04 256.788);
+
+	/* Typography — system stacks, so nothing depends on a CDN or a webfont build. */
+	--app-font-sans: system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
+	--app-font-serif: Georgia, 'Iowan Old Style', 'Times New Roman', serif;
+	--app-font-mono: ui-monospace, 'SF Mono', 'Cascadia Mono', Menlo, Consolas, monospace;
+
+	/* Reading Room — light */
+	--background: #e9ebee;
+	--foreground: #1b1f24;
+	--card: #fbfbfc;
+	--card-foreground: #1b1f24;
+	--popover: #fbfbfc;
+	--popover-foreground: #1b1f24;
+	--primary: #1f5f5b;
+	--primary-foreground: #f2f7f6;
+	--secondary: #dfe3e8;
+	--secondary-foreground: #1b1f24;
+	--muted: #e2e5e9;
+	--muted-foreground: #7d858f;
+	--accent: #d7e5e3;
+	--accent-foreground: #164743;
+	--destructive: #a6402f;
+	--border: #d2d6dc;
+	--input: #d2d6dc;
+	--ring: #1f5f5b;
+
+	/* Semantic — deliberately not the accent, so state never reads as branding. */
+	--success: #2f7d4f;
+	--success-foreground: #f2f7f6;
+	--flag: #b08a1e;
+	--star: #c79a25;
+
+	--chart-1: #1f5f5b;
+	--chart-2: #2f7d4f;
+	--chart-3: #b08a1e;
+	--chart-4: #4c6b8a;
+	--chart-5: #a6402f;
+
+	--sidebar: #e2e5e9;
+	--sidebar-foreground: #1b1f24;
+	--sidebar-primary: #1f5f5b;
+	--sidebar-primary-foreground: #f2f7f6;
+	--sidebar-accent: #d7e5e3;
+	--sidebar-accent-foreground: #164743;
+	--sidebar-border: #d2d6dc;
+	--sidebar-ring: #1f5f5b;
 }
 
 .dark {
-	--background: oklch(0.129 0.042 264.695);
-	--foreground: oklch(0.984 0.003 247.858);
-	--card: oklch(0.208 0.042 265.755);
-	--card-foreground: oklch(0.984 0.003 247.858);
-	--popover: oklch(0.208 0.042 265.755);
-	--popover-foreground: oklch(0.984 0.003 247.858);
-	--primary: oklch(0.929 0.013 255.508);
-	--primary-foreground: oklch(0.208 0.042 265.755);
-	--secondary: oklch(0.279 0.041 260.031);
-	--secondary-foreground: oklch(0.984 0.003 247.858);
-	--muted: oklch(0.279 0.041 260.031);
-	--muted-foreground: oklch(0.704 0.04 256.788);
-	--accent: oklch(0.279 0.041 260.031);
-	--accent-foreground: oklch(0.984 0.003 247.858);
-	--destructive: oklch(0.704 0.191 22.216);
-	--border: oklch(1 0 0 / 10%);
-	--input: oklch(1 0 0 / 15%);
-	--ring: oklch(0.551 0.027 264.364);
-	--chart-1: oklch(0.488 0.243 264.376);
-	--chart-2: oklch(0.696 0.17 162.48);
-	--chart-3: oklch(0.769 0.188 70.08);
-	--chart-4: oklch(0.627 0.265 303.9);
-	--chart-5: oklch(0.645 0.246 16.439);
-	--sidebar: oklch(0.208 0.042 265.755);
-	--sidebar-foreground: oklch(0.984 0.003 247.858);
-	--sidebar-primary: oklch(0.488 0.243 264.376);
-	--sidebar-primary-foreground: oklch(0.984 0.003 247.858);
-	--sidebar-accent: oklch(0.279 0.041 260.031);
-	--sidebar-accent-foreground: oklch(0.984 0.003 247.858);
-	--sidebar-border: oklch(1 0 0 / 10%);
-	--sidebar-ring: oklch(0.551 0.027 264.364);
+	/* Reading Room — dark */
+	--background: #15191b;
+	--foreground: #e6eae9;
+	--card: #1d2226;
+	--card-foreground: #e6eae9;
+	--popover: #1d2226;
+	--popover-foreground: #e6eae9;
+	--primary: #6fbab0;
+	--primary-foreground: #0e1a19;
+	--secondary: #232a2d;
+	--secondary-foreground: #e6eae9;
+	--muted: #232a2d;
+	--muted-foreground: #78868a;
+	--accent: #1b3330;
+	--accent-foreground: #9fd8d0;
+	--destructive: #e0796b;
+	--border: #2a3034;
+	--input: #2a3034;
+	--ring: #6fbab0;
+
+	--success: #4fa97a;
+	--success-foreground: #0e1a19;
+	--flag: #d4a63a;
+	--star: #e5b84b;
+
+	--chart-1: #6fbab0;
+	--chart-2: #4fa97a;
+	--chart-3: #d4a63a;
+	--chart-4: #7f9dc0;
+	--chart-5: #e0796b;
+
+	--sidebar: #111517;
+	--sidebar-foreground: #e6eae9;
+	--sidebar-primary: #6fbab0;
+	--sidebar-primary-foreground: #0e1a19;
+	--sidebar-accent: #1b3330;
+	--sidebar-accent-foreground: #9fd8d0;
+	--sidebar-border: #2a3034;
+	--sidebar-ring: #6fbab0;
 }
 
 @theme inline {
+	--font-sans: var(--app-font-sans);
+	--font-serif: var(--app-font-serif);
+	--font-mono: var(--app-font-mono);
 	--radius-sm: calc(var(--radius) - 4px);
 	--radius-md: calc(var(--radius) - 2px);
 	--radius-lg: var(--radius);
@@ -95,6 +121,10 @@
 	--color-accent: var(--accent);
 	--color-accent-foreground: var(--accent-foreground);
 	--color-destructive: var(--destructive);
+	--color-success: var(--success);
+	--color-success-foreground: var(--success-foreground);
+	--color-flag: var(--flag);
+	--color-star: var(--star);
 	--color-border: var(--border);
 	--color-input: var(--input);
 	--color-ring: var(--ring);
diff --git a/frontend/src/app.d.ts b/frontend/src/app.d.ts
index 6cafeb6..8b24ae2 100644
--- a/frontend/src/app.d.ts
+++ b/frontend/src/app.d.ts
@@ -2,7 +2,8 @@
 // for information about these interfaces
 
 import type { ApiClient } from '$lib/server/api';
-import type { User } from 'lucide-svelte';
+import type { User } from '$lib/server/auth';
+import type { ThemeConfig } from '$lib/theme/presets';
 
 declare global {
 	namespace App {
@@ -11,6 +12,7 @@ declare global {
 			authToken: string | null;
 			api: ApiClient;
 			user: User;
+			theme: ThemeConfig;
 		}
 		// interface PageData {}
 		// interface PageState {}
diff --git a/frontend/src/app.html b/frontend/src/app.html
index 06a6722..9a35db9 100644
--- a/frontend/src/app.html
+++ b/frontend/src/app.html
@@ -4,6 +4,7 @@
 		<meta charset="utf-8" />
 		<meta name="viewport" content="width=device-width, initial-scale=1" />
 		%sveltekit.head%
+		<!--theme-->
 	</head>
 	<body data-sveltekit-preload-data="hover" class="overflow-hidden">
 		<div style="display: contents">%sveltekit.body%</div>
diff --git a/frontend/src/hooks.server.ts b/frontend/src/hooks.server.ts
index f7ac519..af0d5c5 100644
--- a/frontend/src/hooks.server.ts
+++ b/frontend/src/hooks.server.ts
@@ -1,5 +1,6 @@
 import { ApiClient } from '$lib/server/api';
 import { validateToken } from '$lib/server/auth';
+import { THEME_COOKIE, parseThemeCookie, themeToCss } from '$lib/theme/presets';
 import { redirect, type Handle } from '@sveltejs/kit';
 import { sequence } from '@sveltejs/kit/hooks';
 
@@ -34,4 +35,28 @@ const protectedRoutesHandle: Handle = async ({ event, resolve }) => {
 	return resolve(event);
 };
 
-export const handle = sequence(authHandle, protectedRoutesHandle);
+/**
+ * Inline the stored theme into the document head.
+ *
+ * The palette has to be in the very first byte of HTML the browser paints,
+ * otherwise every page load flashes the default theme before hydration swaps
+ * it. The `<!--theme-->` placeholder in app.html is the injection point.
+ */
+const themeHandle: Handle = async ({ event, resolve }) => {
+	const config = parseThemeCookie(event.cookies.get(THEME_COOKIE));
+	event.locals.theme = config;
+
+	return resolve(event, {
+		// The placeholder comment is kept, not replaced. Svelte 5 uses HTML
+		// comments as hydration markers, so SvelteKit warns when a chunk comes
+		// back with fewer comments than it went in with — removing this one is
+		// enough to trip that check.
+		transformPageChunk: ({ html }) =>
+			html.replace(
+				'<!--theme-->',
+				`<!--theme--><style id="chitai-theme">${themeToCss(config)}</style>`
+			)
+	});
+};
+
+export const handle = sequence(themeHandle, authHandle, protectedRoutesHandle);
diff --git a/frontend/src/lib/theme/presets.ts b/frontend/src/lib/theme/presets.ts
new file mode 100644
index 0000000..c2a4c8b
--- /dev/null
+++ b/frontend/src/lib/theme/presets.ts
@@ -0,0 +1,346 @@
+/**
+ * Theme presets and the definition of what the editor may change.
+ *
+ * A preset supplies a full palette for both modes. The editor writes sparse
+ * overrides on top, per mode, so customising the dark palette never disturbs
+ * the light one.
+ */
+
+export type Mode = 'light' | 'dark';
+
+/** CSS custom properties the editor exposes, in the order it shows them. */
+export const COLOR_TOKENS = [
+	{ key: 'background', label: 'Background', hint: 'Page behind everything' },
+	{ key: 'foreground', label: 'Text', hint: 'Default body text' },
+	{ key: 'card', label: 'Surface', hint: 'Cards, rows, popovers' },
+	{ key: 'primary', label: 'Primary', hint: 'Buttons and active states' },
+	{ key: 'muted-foreground', label: 'Muted text', hint: 'Counts, captions, metadata' },
+	{ key: 'border', label: 'Border', hint: 'Rules and outlines' },
+	{ key: 'sidebar', label: 'Sidebar', hint: 'Sidebar and toolbar ground' },
+	{ key: 'success', label: 'Finished', hint: 'Completed reading progress' },
+	{ key: 'flag', label: 'Active filter', hint: 'The dot on Filter and Sort' },
+	{ key: 'star', label: 'Favourite', hint: 'Stars and selection rings' },
+	{ key: 'destructive', label: 'Destructive', hint: 'Delete actions' }
+] as const;
+
+export type ColorTokenKey = (typeof COLOR_TOKENS)[number]['key'];
+
+export const FONT_TOKENS = [
+	{ key: 'app-font-sans', label: 'Interface' },
+	{ key: 'app-font-serif', label: 'Titles' },
+	{ key: 'app-font-mono', label: 'Numbers and labels' }
+] as const;
+
+export type FontTokenKey = (typeof FONT_TOKENS)[number]['key'];
+
+/** System stacks only — no webfont fetch, so nothing can silently fall back. */
+export const FONT_STACKS: { label: string; value: string }[] = [
+	{
+		label: 'System UI',
+		value: "system-ui, -apple-system, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif"
+	},
+	{ label: 'Georgia', value: "Georgia, 'Iowan Old Style', 'Times New Roman', serif" },
+	{ label: 'Times', value: "'Times New Roman', Times, serif" },
+	{ label: 'Palatino', value: "'Palatino Linotype', Palatino, 'Book Antiqua', serif" },
+	{ label: 'Helvetica', value: "'Helvetica Neue', Helvetica, Arial, sans-serif" },
+	{ label: 'Verdana', value: 'Verdana, Geneva, sans-serif' },
+	{ label: 'Monospace', value: "ui-monospace, 'SF Mono', 'Cascadia Mono', Menlo, Consolas, monospace" }
+];
+
+export type Palette = Record<string, string>;
+
+export interface Preset {
+	id: string;
+	name: string;
+	description: string;
+	radius: string;
+	fonts: Record<FontTokenKey, string>;
+	light: Palette;
+	dark: Palette;
+}
+
+const SANS = FONT_STACKS[0].value;
+const SERIF = FONT_STACKS[1].value;
+const MONO = FONT_STACKS[6].value;
+
+export const READING_ROOM: Preset = {
+	id: 'reading-room',
+	name: 'Reading Room',
+	description: 'Cool grey paper, one teal accent, serif titles.',
+	radius: '0.625rem',
+	fonts: { 'app-font-sans': SANS, 'app-font-serif': SERIF, 'app-font-mono': MONO },
+	light: {
+		background: '#e9ebee',
+		foreground: '#1b1f24',
+		card: '#fbfbfc',
+		'card-foreground': '#1b1f24',
+		popover: '#fbfbfc',
+		'popover-foreground': '#1b1f24',
+		primary: '#1f5f5b',
+		'primary-foreground': '#f2f7f6',
+		secondary: '#dfe3e8',
+		'secondary-foreground': '#1b1f24',
+		muted: '#e2e5e9',
+		'muted-foreground': '#7d858f',
+		accent: '#d7e5e3',
+		'accent-foreground': '#164743',
+		destructive: '#a6402f',
+		border: '#d2d6dc',
+		input: '#d2d6dc',
+		ring: '#1f5f5b',
+		success: '#2f7d4f',
+		'success-foreground': '#f2f7f6',
+		flag: '#b08a1e',
+		star: '#c79a25',
+		sidebar: '#e2e5e9',
+		'sidebar-foreground': '#1b1f24',
+		'sidebar-primary': '#1f5f5b',
+		'sidebar-primary-foreground': '#f2f7f6',
+		'sidebar-accent': '#d7e5e3',
+		'sidebar-accent-foreground': '#164743',
+		'sidebar-border': '#d2d6dc',
+		'sidebar-ring': '#1f5f5b'
+	},
+	dark: {
+		background: '#15191b',
+		foreground: '#e6eae9',
+		card: '#1d2226',
+		'card-foreground': '#e6eae9',
+		popover: '#1d2226',
+		'popover-foreground': '#e6eae9',
+		primary: '#6fbab0',
+		'primary-foreground': '#0e1a19',
+		secondary: '#232a2d',
+		'secondary-foreground': '#e6eae9',
+		muted: '#232a2d',
+		'muted-foreground': '#78868a',
+		accent: '#1b3330',
+		'accent-foreground': '#9fd8d0',
+		destructive: '#e0796b',
+		border: '#2a3034',
+		input: '#2a3034',
+		ring: '#6fbab0',
+		success: '#4fa97a',
+		'success-foreground': '#0e1a19',
+		flag: '#d4a63a',
+		star: '#e5b84b',
+		sidebar: '#111517',
+		'sidebar-foreground': '#e6eae9',
+		'sidebar-primary': '#6fbab0',
+		'sidebar-primary-foreground': '#0e1a19',
+		'sidebar-accent': '#1b3330',
+		'sidebar-accent-foreground': '#9fd8d0',
+		'sidebar-border': '#2a3034',
+		'sidebar-ring': '#6fbab0'
+	}
+};
+
+export const STACKS: Preset = {
+	id: 'stacks',
+	name: 'Stacks',
+	description: 'Warm near-black with brass. Covers do the talking.',
+	radius: '0.25rem',
+	fonts: { 'app-font-sans': SANS, 'app-font-serif': SERIF, 'app-font-mono': MONO },
+	light: {
+		...READING_ROOM.light,
+		background: '#f2efe9',
+		foreground: '#221e1a',
+		card: '#fbf9f5',
+		'card-foreground': '#221e1a',
+		popover: '#fbf9f5',
+		'popover-foreground': '#221e1a',
+		primary: '#8a6a15',
+		'primary-foreground': '#fbf9f5',
+		secondary: '#e7e1d6',
+		'secondary-foreground': '#221e1a',
+		muted: '#e7e1d6',
+		'muted-foreground': '#7d7266',
+		accent: '#efe6d2',
+		'accent-foreground': '#5c4708',
+		border: '#ddd5c7',
+		input: '#ddd5c7',
+		ring: '#8a6a15',
+		flag: '#8a6a15',
+		star: '#a8801d',
+		sidebar: '#ebe6dc',
+		'sidebar-foreground': '#221e1a',
+		'sidebar-primary': '#8a6a15',
+		'sidebar-primary-foreground': '#fbf9f5',
+		'sidebar-accent': '#efe6d2',
+		'sidebar-accent-foreground': '#5c4708',
+		'sidebar-border': '#ddd5c7',
+		'sidebar-ring': '#8a6a15'
+	},
+	dark: {
+		...READING_ROOM.dark,
+		background: '#141210',
+		foreground: '#ede7de',
+		card: '#221e1a',
+		'card-foreground': '#ede7de',
+		popover: '#221e1a',
+		'popover-foreground': '#ede7de',
+		primary: '#c9a227',
+		'primary-foreground': '#17130a',
+		secondary: '#2c2621',
+		'secondary-foreground': '#ede7de',
+		muted: '#2c2621',
+		'muted-foreground': '#7d7266',
+		accent: '#352c14',
+		'accent-foreground': '#e6c452',
+		border: '#2c2621',
+		input: '#2c2621',
+		ring: '#c9a227',
+		flag: '#c9a227',
+		star: '#e6c452',
+		sidebar: '#100e0c',
+		'sidebar-foreground': '#ede7de',
+		'sidebar-primary': '#c9a227',
+		'sidebar-primary-foreground': '#17130a',
+		'sidebar-accent': '#352c14',
+		'sidebar-accent-foreground': '#e6c452',
+		'sidebar-border': '#2c2621',
+		'sidebar-ring': '#c9a227'
+	}
+};
+
+export const SLATE: Preset = {
+	id: 'slate',
+	name: 'Slate',
+	description: 'The original shadcn palette, kept for comparison.',
+	radius: '0.625rem',
+	fonts: { 'app-font-sans': SANS, 'app-font-serif': SANS, 'app-font-mono': MONO },
+	light: {
+		...READING_ROOM.light,
+		background: '#ffffff',
+		foreground: '#020617',
+		card: '#ffffff',
+		'card-foreground': '#020617',
+		popover: '#ffffff',
+		'popover-foreground': '#020617',
+		primary: '#1e293b',
+		'primary-foreground': '#f8fafc',
+		secondary: '#f1f5f9',
+		'secondary-foreground': '#1e293b',
+		muted: '#f1f5f9',
+		'muted-foreground': '#64748b',
+		accent: '#f1f5f9',
+		'accent-foreground': '#1e293b',
+		destructive: '#dc2626',
+		border: '#e2e8f0',
+		input: '#e2e8f0',
+		ring: '#94a3b8',
+		success: '#16a34a',
+		flag: '#eab308',
+		star: '#fde047',
+		sidebar: '#f8fafc',
+		'sidebar-foreground': '#020617',
+		'sidebar-primary': '#1e293b',
+		'sidebar-primary-foreground': '#f8fafc',
+		'sidebar-accent': '#f1f5f9',
+		'sidebar-accent-foreground': '#1e293b',
+		'sidebar-border': '#e2e8f0',
+		'sidebar-ring': '#94a3b8'
+	},
+	dark: {
+		...READING_ROOM.dark,
+		background: '#020617',
+		foreground: '#f8fafc',
+		card: '#1e293b',
+		'card-foreground': '#f8fafc',
+		popover: '#1e293b',
+		'popover-foreground': '#f8fafc',
+		primary: '#e2e8f0',
+		'primary-foreground': '#1e293b',
+		secondary: '#334155',
+		'secondary-foreground': '#f8fafc',
+		muted: '#334155',
+		'muted-foreground': '#94a3b8',
+		accent: '#334155',
+		'accent-foreground': '#f8fafc',
+		destructive: '#f87171',
+		border: '#334155',
+		input: '#334155',
+		ring: '#64748b',
+		success: '#4ade80',
+		flag: '#eab308',
+		star: '#fde047',
+		sidebar: '#1e293b',
+		'sidebar-foreground': '#f8fafc',
+		'sidebar-primary': '#60a5fa',
+		'sidebar-primary-foreground': '#f8fafc',
+		'sidebar-accent': '#334155',
+		'sidebar-accent-foreground': '#f8fafc',
+		'sidebar-border': '#334155',
+		'sidebar-ring': '#64748b'
+	}
+};
+
+export const PRESETS: Preset[] = [READING_ROOM, STACKS, SLATE];
+
+export const DEFAULT_PRESET_ID = READING_ROOM.id;
+
+export function getPreset(id: string | undefined): Preset {
+	return PRESETS.find((p) => p.id === id) ?? READING_ROOM;
+}
+
+/** What we persist in the cookie. Sparse by design — presets carry the rest. */
+export interface ThemeConfig {
+	preset: string;
+	radius?: string;
+	fonts?: Partial<Record<FontTokenKey, string>>;
+	light?: Palette;
+	dark?: Palette;
+}
+
+export const THEME_COOKIE = 'chitai-theme';
+
+export function parseThemeCookie(raw: string | undefined | null): ThemeConfig {
+	if (!raw) return { preset: DEFAULT_PRESET_ID };
+	try {
+		const parsed = JSON.parse(decodeURIComponent(raw));
+		if (!parsed || typeof parsed !== 'object') return { preset: DEFAULT_PRESET_ID };
+		return { ...parsed, preset: typeof parsed.preset === 'string' ? parsed.preset : DEFAULT_PRESET_ID };
+	} catch {
+		return { preset: DEFAULT_PRESET_ID };
+	}
+}
+
+/** Merge a config over its preset to get the palette actually in force. */
+export function resolvePalette(config: ThemeConfig, mode: Mode): Palette {
+	const preset = getPreset(config.preset);
+	return { ...preset[mode], ...(config[mode] ?? {}) };
+}
+
+export function resolveRadius(config: ThemeConfig): string {
+	return config.radius ?? getPreset(config.preset).radius;
+}
+
+export function resolveFonts(config: ThemeConfig): Record<FontTokenKey, string> {
+	return { ...getPreset(config.preset).fonts, ...(config.fonts ?? {}) };
+}
+
+/**
+ * Render a theme as CSS. Used on the server to inline the theme into the
+ * document head, so the first paint is already correct.
+ *
+ * The doubled `:root:root` is deliberate. app.css declares the same custom
+ * properties on plain `:root`, which has identical specificity, so whichever
+ * stylesheet comes last would win — and in dev Vite appends app.css to the end
+ * of <head>, after this tag. Doubling the selector raises specificity to
+ * (0,2,0) so the stored theme wins on merit rather than on load order.
+ */
+export function themeToCss(config: ThemeConfig): string {
+	const decl = (palette: Palette) =>
+		Object.entries(palette)
+			.map(([k, v]) => `--${k}:${v};`)
+			.join('');
+
+	const fonts = Object.entries(resolveFonts(config))
+		.map(([k, v]) => `--${k}:${v};`)
+		.join('');
+
+	return (
+		`:root:root{--radius:${resolveRadius(config)};${fonts}${decl(resolvePalette(config, 'light'))}}` +
+		`:root:root.dark{${decl(resolvePalette(config, 'dark'))}}`
+	);
+}
diff --git a/frontend/src/lib/theme/theme.svelte.ts b/frontend/src/lib/theme/theme.svelte.ts
new file mode 100644
index 0000000..3495df5
--- /dev/null
+++ b/frontend/src/lib/theme/theme.svelte.ts
@@ -0,0 +1,133 @@
+import { getContext, setContext } from 'svelte';
+import { browser } from '$app/environment';
+import { mode } from 'mode-watcher';
+import {
+	DEFAULT_PRESET_ID,
+	THEME_COOKIE,
+	getPreset,
+	resolveFonts,
+	resolvePalette,
+	resolveRadius,
+	type ColorTokenKey,
+	type FontTokenKey,
+	type Mode,
+	type ThemeConfig
+} from './presets';
+
+/**
+ * Live theme state.
+ *
+ * The server has already inlined the stored theme into the document head, so
+ * this class only takes over once the user edits something: it writes the
+ * changed custom properties onto <html> and persists the config to a cookie.
+ */
+export class ThemeState {
+	config = $state<ThemeConfig>({ preset: DEFAULT_PRESET_ID });
+
+	/** Which palette the editor is currently writing to. */
+	readonly mode = $derived<Mode>(mode.current === 'dark' ? 'dark' : 'light');
+
+	readonly preset = $derived(getPreset(this.config.preset));
+	readonly palette = $derived(resolvePalette(this.config, this.mode));
+	readonly radius = $derived(resolveRadius(this.config));
+	readonly fonts = $derived(resolveFonts(this.config));
+
+	readonly isCustomised = $derived(
+		Boolean(
+			this.config.radius ||
+				this.config.fonts ||
+				Object.keys(this.config.light ?? {}).length ||
+				Object.keys(this.config.dark ?? {}).length
+		)
+	);
+
+	/**
+	 * True once this session has written inline custom properties onto <html>.
+	 * Until then the server-injected stylesheet is the only source of theme, and
+	 * it already covers both modes — so we touch nothing and there is no repaint.
+	 */
+	private touched = false;
+
+	constructor(config: ThemeConfig) {
+		this.config = config;
+	}
+
+	setPreset(id: string) {
+		// Switching preset discards overrides; keeping them across palettes
+		// produces colour combinations nobody chose.
+		this.config = { preset: id };
+		this.apply();
+	}
+
+	setColor(token: ColorTokenKey | string, value: string) {
+		const mode = this.mode;
+		this.config = { ...this.config, [mode]: { ...(this.config[mode] ?? {}), [token]: value } };
+		this.apply();
+	}
+
+	setRadius(value: string) {
+		this.config = { ...this.config, radius: value };
+		this.apply();
+	}
+
+	setFont(token: FontTokenKey, value: string) {
+		this.config = { ...this.config, fonts: { ...(this.config.fonts ?? {}), [token]: value } };
+		this.apply();
+	}
+
+	reset() {
+		this.config = { preset: this.config.preset };
+		this.apply();
+	}
+
+	/** Push the resolved theme onto the document and persist it. */
+	apply() {
+		if (!browser) return;
+		this.touched = true;
+
+		const root = document.documentElement;
+		root.style.setProperty('--radius', this.radius);
+
+		for (const [key, value] of Object.entries(this.fonts)) {
+			root.style.setProperty(`--${key}`, value);
+		}
+
+		// Inline styles sit on :root and so apply in both modes. Write the
+		// palette for the mode being displayed and rewrite it on mode change.
+		for (const [key, value] of Object.entries(this.palette)) {
+			root.style.setProperty(`--${key}`, value);
+		}
+
+		this.persist();
+	}
+
+	/**
+	 * Re-apply after a light/dark switch, since inline vars are mode-blind.
+	 * No-op until something has actually been edited — see `touched`.
+	 */
+	syncMode() {
+		if (!browser || !this.touched) return;
+		const palette = resolvePalette(this.config, this.mode);
+		for (const [key, value] of Object.entries(palette)) {
+			document.documentElement.style.setProperty(`--${key}`, value);
+		}
+	}
+
+	private persist() {
+		if (!browser) return;
+		const value = encodeURIComponent(JSON.stringify(this.config));
+		// One year, root path, lax — same shape as the auth cookie minus httpOnly,
+		// because the editor has to be able to read and rewrite it client-side.
+		document.cookie = `${THEME_COOKIE}=${value};path=/;max-age=31536000;samesite=lax`;
+	}
+}
+
+const THEME_KEY = Symbol('THEME');
+
+export function setThemeState(config: ThemeConfig) {
+	return setContext(THEME_KEY, new ThemeState(config));
+}
+
+export function getThemeState() {
+	return getContext<ReturnType<typeof setThemeState>>(THEME_KEY);
+}
diff --git a/frontend/src/routes/(root)/+layout.server.ts b/frontend/src/routes/(root)/+layout.server.ts
index 15cdb65..3e700f6 100644
--- a/frontend/src/routes/(root)/+layout.server.ts
+++ b/frontend/src/routes/(root)/+layout.server.ts
@@ -1,9 +1,11 @@
 import { listLibraries } from '$lib/api/library.remote.js';
 
-export const load = async ({ depends }) => {
+export const load = async ({ depends, locals }) => {
 	depends('app:libraries');
 
 	return {
-		libraries: await listLibraries({ pageSize: 100 })
+		libraries: await listLibraries({ pageSize: 100 }),
+		theme: locals.theme,
+		user: locals.user
 	};
 };
diff --git a/frontend/src/routes/(root)/+layout.svelte b/frontend/src/routes/(root)/+layout.svelte
index a8d216e..6e7a831 100644
--- a/frontend/src/routes/(root)/+layout.svelte
+++ b/frontend/src/routes/(root)/+layout.svelte
@@ -8,17 +8,36 @@
 	import { setBookOperationsState } from '$lib/state/bookOperations.svelte';
 	import { setBookshelfState } from '$lib/state/bookshelf.svelte';
 	import { setLibraryState } from '$lib/state/library.svelte.js';
+	import { setThemeState } from '$lib/theme/theme.svelte';
+	import type { ThemeConfig } from '$lib/theme/presets';
 
 	import { ModeWatcher } from 'mode-watcher';
-	import type { Snippet } from 'svelte';
+	import { untrack, type Snippet } from 'svelte';
 	import { Toaster } from 'svelte-sonner';
 
-	let { data, children }: { data: { libraries: PaginatedResponse<Library> }; children: Snippet } =
-		$props();
+	let {
+		data,
+		children
+	}: {
+		data: { libraries: PaginatedResponse<Library>; theme: ThemeConfig; user: { email: string } };
+		children: Snippet;
+	} = $props();
 
-	const libraryState = setLibraryState(data.libraries.items);
+	// These states are seeded once and then own their own data — LibraryState has
+	// updateLibraries() for later syncing, and ThemeState owns the config after
+	// init. `untrack` says that explicitly, instead of silently capturing the
+	// initial value and warning about it.
+	const libraryState = setLibraryState(untrack(() => data.libraries.items));
 	const bookshelfState = setBookshelfState();
 	const bookOps = setBookOperationsState(libraryState.activeLibrary!.id);
+	const theme = setThemeState(untrack(() => data.theme));
+
+	// Inline custom properties live on :root and so are mode-blind. When the
+	// light/dark switch flips, rewrite them for the mode now showing.
+	$effect(() => {
+		theme.mode;
+		theme.syncMode();
+	});
 </script>
 
 <Toaster />
@@ -27,7 +46,7 @@
 <div class="[--header-height:calc(--spacing(14))]">
 	<Sidebar.Provider class="flex flex-col">
 		<div class="flex h-screen">
-			<AppSidebar />
+			<AppSidebar user={data.user} />
 			<Sidebar.Inset class="flex min-w-0 flex-1 flex-col overflow-x-hidden">
 				<SiteHeader />
 				<div class="mt-4 flex-1 overflow-hidden p-4 pt-0">