Studio: pick the accent foreground by contrast, not a luminance cutoff

readableForeground returned white for anything under 0.45 luminance, so
mid-tone accents got an unreadable label: #22c55e scored 2.28:1 on white
where the dark ink scores 8.11:1. Compare the two contrast ratios and
take the winner, which needs no threshold constant.

studio/frontend/src/features/settings/stores/appearance-custom-store.ts

@@ -395,8 +395,27 @@ function hexLuminance(hex: string): number {
   return 0.2126 * channel(1) + 0.7152 * channel(3) + 0.0722 * channel(5);
 }
 
+const FOREGROUND_DARK = "#111417";
+const FOREGROUND_LIGHT = "#ffffff";
+
+/** WCAG contrast ratio between two relative luminances. */
+function contrastRatio(a: number, b: number): number {
+  const [high, low] = a >= b ? [a, b] : [b, a];
+  return (high + 0.05) / (low + 0.05);
+}
+
+/**
+ * Whichever foreground actually contrasts more. A fixed luminance threshold
+ * put white on mid-tone accents: #22c55e scored 2.28:1 on white against
+ * 8.11:1 on the dark ink. The crossover for this pair is near 0.19, but
+ * comparing the ratios needs no constant at all.
+ */
 function readableForeground(hex: string): string {
-  return hexLuminance(hex) > 0.45 ? "#111417" : "#ffffff";
+  const accent = hexLuminance(hex);
+  return contrastRatio(accent, hexLuminance(FOREGROUND_DARK)) >=
+    contrastRatio(accent, hexLuminance(FOREGROUND_LIGHT))
+    ? FOREGROUND_DARK
+    : FOREGROUND_LIGHT;
 }
 
 /**

studio/frontend/tests/appearance-accent-vars.test.ts

@@ -63,6 +63,53 @@ test("both foregrounds follow the accent so button labels stay readable", () =>
   assert.notEqual(vars.get("--primary-foreground"), onDark);
 });
 
+/** WCAG relative luminance, independent of the implementation under test. */
+function luminance(hex: string): number {
+  const channel = (index: number) => {
+    const value = Number.parseInt(hex.slice(index, index + 2), 16) / 255;
+    return value <= 0.03928 ? value / 12.92 : ((value + 0.055) / 1.055) ** 2.4;
+  };
+  return 0.2126 * channel(1) + 0.7152 * channel(3) + 0.0722 * channel(5);
+}
+
+function ratio(a: string, b: string): number {
+  const [high, low] = [luminance(a), luminance(b)].sort((x, y) => y - x);
+  return ((high ?? 0) + 0.05) / ((low ?? 0) + 0.05);
+}
+
+test("the foreground is the higher-contrast of the two, not a luminance guess", () => {
+  // Mid-tone accents are the ones a fixed 0.45 cutoff got wrong.
+  for (const accent of [
+    "#22c55e",
+    "#17b88b",
+    "#339cff",
+    "#f59e0b",
+    "#4ade80",
+    "#7c3aed",
+    "#e11d48",
+    "#0d0d0d",
+    "#fde68a",
+    "#ececec",
+  ]) {
+    applyCustomizationToDocument(withAccent(accent), "light");
+    const chosen = vars.get("--primary-foreground") ?? "";
+    const other = chosen === "#ffffff" ? "#111417" : "#ffffff";
+    assert.ok(
+      ratio(accent, chosen) >= ratio(accent, other),
+      `${accent}: picked ${chosen} at ${ratio(accent, chosen).toFixed(2)}:1 over ${other} at ${ratio(accent, other).toFixed(2)}:1`,
+    );
+  }
+});
+
+test("a saturated green label clears WCAG AA instead of failing it", () => {
+  applyCustomizationToDocument(withAccent("#22c55e"), "light");
+  const chosen = vars.get("--primary-foreground") ?? "";
+  assert.ok(
+    ratio("#22c55e", chosen) >= 4.5,
+    `#22c55e on ${chosen} is only ${ratio("#22c55e", chosen).toFixed(2)}:1`,
+  );
+});
+
 test("no accent leaves every palette variable alone", () => {
   applyCustomizationToDocument(withAccent("#7c3aed"), "light");
   applyCustomizationToDocument(withAccent(null), "light");