feat(accent): custom accent themes links, text selection, and focus rings

The accent previously only overrode the folds Primary.* family; links kept the
hardcoded --tc-link blue, ::selection was browser-default, and focus rings were
neutral grey (Other.FocusRing). Now all three derive from the chosen base color:
- --tc-link → accent hex (messages, topics, URL previews)
- ::selection via an injected <style id="lotus-accent-style"> (accent bg +
  WCAG-contrasting text)
- Other.FocusRing → rgba(accent, 0.5)

Deliberately NOT recolored: Secondary.* (doubles as the neutral text/button/
badge palette), Success.* + mention pills (semantic mention/notification green),
scrollbar thumbs (folds styles them per-component; a global rule would only
half-apply). removeCustomAccent() clears everything — no residue when switching
off or to the TDS theme. +2 unit tests (561 total).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/accentColor.test.ts

@@ -9,6 +9,8 @@ import {
   contrastingText,
   varNameFromToken,
   derivePrimaryPalette,
+  deriveAccentExtras,
+  buildAccentCss,
 } from './accentColor';
 
 test('hexToRgb parses 6-digit hex (with/without #, trimmed)', () => {
@@ -66,3 +68,22 @@ test('derivePrimaryPalette produces the full Primary token set', () => {
   assert.match(palette.MainHover, /^#[0-9a-f]{6}$/);
   assert.match(palette.MainActive, /^#[0-9a-f]{6}$/);
 });
+
+test('deriveAccentExtras derives focus ring, link and selection from one base', () => {
+  const base = { r: 255, g: 136, b: 0 };
+  const extras = deriveAccentExtras(base);
+  // focus ring keeps the translucent character in the accent hue
+  assert.equal(extras.focusRing, 'rgba(255, 136, 0, 0.5)');
+  // link + selection background are the solid base hex
+  assert.equal(extras.link, '#ff8800');
+  assert.equal(extras.selectionBg, '#ff8800');
+  // selection text is WCAG-aware contrasting text over the base
+  assert.equal(extras.selectionText, contrastingText(base));
+});
+
+test('buildAccentCss emits selection rules using the derived palette', () => {
+  const base = { r: 0, g: 0, b: 0 };
+  const css = buildAccentCss(base);
+  assert.match(css, /::selection\{background:#000000;color:#fff;\}/);
+  assert.match(css, /::-moz-selection\{background:#000000;color:#fff;\}/);
+});

src/app/utils/accentColor.ts

@@ -74,6 +74,45 @@ const PRIMARY_TOKENS: Record<string, string> = {
   OnContainer: color.Primary.OnContainer,
 };
 
+// The neutral focus-ring token folds uses for the outline on inputs, buttons,
+// switches, checkboxes and radios. Its default is a semi-transparent grey/black,
+// so tinting it in the accent hue themes every focus ring without touching the
+// neutral Secondary family (see below). We keep the same translucent character
+// so it reads as a ring rather than a fill.
+const FOCUS_RING_TOKEN = color.Other.FocusRing;
+
+// `--tc-link` is the global anchor color (index.css `a { color: var(--tc-link) }`);
+// overriding it themes plain links inside messages, room topics and URL previews.
+const LINK_VAR = '--tc-link';
+
+// Injected stylesheet id — carries rules that cannot be expressed as a single
+// CSS variable (currently text ::selection).
+const ACCENT_STYLE_ID = 'lotus-accent-style';
+
+export type AccentExtras = {
+  focusRing: string;
+  link: string;
+  selectionBg: string;
+  selectionText: string;
+};
+
+// Derive the extra (non-Primary) accent values from the single base color, using
+// the same helpers as the Primary palette so everything stays in one hue.
+export const deriveAccentExtras = (base: Rgb): AccentExtras => ({
+  focusRing: rgba(base, 0.5),
+  link: rgbToHex(base),
+  selectionBg: rgbToHex(base),
+  selectionText: contrastingText(base),
+});
+
+// Build the injected stylesheet body. Selection uses a solid accent fill with
+// WCAG-aware contrasting text so highlighted text stays readable.
+export const buildAccentCss = (base: Rgb): string => {
+  const { selectionBg, selectionText } = deriveAccentExtras(base);
+  const selection = `background:${selectionBg};color:${selectionText};`;
+  return `::selection{${selection}}::-moz-selection{${selection}}`;
+};
+
 // Derive the 10 Primary sub-token values from a single chosen base color.
 export const derivePrimaryPalette = (base: Rgb): Record<string, string> => {
   const baseHex = rgbToHex(base);
@@ -96,22 +135,46 @@ export const derivePrimaryPalette = (base: Rgb): Record<string, string> => {
 };
 
 // Apply a custom accent color by overriding the folds Primary CSS variables on
-// `document.body`. Returns true when applied, false when the input is invalid.
+// `document.body`, tinting the focus-ring and link vars, and injecting a small
+// stylesheet for text selection. Returns true when applied, false when the input
+// is invalid.
 export const applyCustomAccent = (hex: string): boolean => {
   const base = hexToRgb(hex);
   if (!base) return false;
+
   const palette = derivePrimaryPalette(base);
   Object.entries(PRIMARY_TOKENS).forEach(([key, token]) => {
     const varName = varNameFromToken(token);
     if (varName) document.body.style.setProperty(varName, palette[key]);
   });
+
+  const extras = deriveAccentExtras(base);
+  const focusRingVar = varNameFromToken(FOCUS_RING_TOKEN);
+  if (focusRingVar) document.body.style.setProperty(focusRingVar, extras.focusRing);
+  document.body.style.setProperty(LINK_VAR, extras.link);
+
+  let styleEl = document.getElementById(ACCENT_STYLE_ID) as HTMLStyleElement | null;
+  if (!styleEl) {
+    styleEl = document.createElement('style');
+    styleEl.id = ACCENT_STYLE_ID;
+    document.head.appendChild(styleEl);
+  }
+  styleEl.textContent = buildAccentCss(base);
+
   return true;
 };
 
 // Remove all custom accent overrides, reverting to the active theme's defaults.
+// Idempotent — safe to call even when nothing was applied.
 export const removeCustomAccent = (): void => {
   Object.values(PRIMARY_TOKENS).forEach((token) => {
     const varName = varNameFromToken(token);
     if (varName) document.body.style.removeProperty(varName);
   });
+
+  const focusRingVar = varNameFromToken(FOCUS_RING_TOKEN);
+  if (focusRingVar) document.body.style.removeProperty(focusRingVar);
+  document.body.style.removeProperty(LINK_VAR);
+
+  document.getElementById(ACCENT_STYLE_ID)?.remove();
 };