fix: add dark mode support for Carta markdown editor

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/routes/layout.css

@@ -123,4 +123,20 @@
 /* Revert Tailwind's Preflight resets inside Carta's markdown renderer */
 .carta-renderer * {
   all: revert;
+}
+
+/* Carta editor dark mode - swap to dark variants */
+.dark .carta-theme__default {
+  --border-color: var(--border-color-dark);
+  --selection-color: var(--selection-color-dark);
+  --focus-outline: var(--focus-outline-dark);
+  --hover-color: var(--hover-color-dark);
+  --caret-color: var(--caret-color-dark);
+  --text-color: var(--text-color-dark);
+}
+
+/* Carta syntax highlighting dark mode */
+.dark .shiki,
+.dark .shiki span {
+  color: var(--shiki-dark) !important;
 }