fix(mornye): preserve contrast with custom accent colors

lib/theme/mornye_theme.dart

@@ -66,7 +66,9 @@ class MornyeTheme extends ThemeExtension<MornyeTheme> {
       MornyeAccent.purple => CupertinoColors.systemPurple,
       MornyeAccent.pink => CupertinoColors.systemPink,
     };
-    return dark ? color.darkColor : color.color;
+    // Accents also label links and small controls on white surfaces.
+    if (!dark && accent == MornyeAccent.green) return const Color(0xff24863d);
+    return dark ? color.darkColor : color.highContrastColor;
   }
 
   static final tokens = AppTokens.standard.copyWith(
@@ -169,7 +171,11 @@ class MornyeTheme extends ThemeExtension<MornyeTheme> {
           brightness: brightness,
         ).copyWith(
           primary: accent,
-          onPrimary: Colors.white,
+          onPrimary:
+              selectedAccent == MornyeAccent.red ||
+                  accent.computeLuminance() < 0.179
+              ? Colors.white
+              : Colors.black,
           primaryContainer: grouped,
           onPrimaryContainer: accent,
           secondary: accent,

test/appearance_settings_test.dart

@@ -10,6 +10,35 @@ import 'package:spotiflac_android/theme/dynamic_color_wrapper.dart';
 import 'package:spotiflac_android/theme/mornye_theme.dart';
 
 void main() {
+  test('custom accents keep text readable in both appearances', () {
+    double contrast(Color first, Color second) {
+      final a = first.computeLuminance();
+      final b = second.computeLuminance();
+      return a > b ? (a + 0.05) / (b + 0.05) : (b + 0.05) / (a + 0.05);
+    }
+
+    for (final brightness in Brightness.values) {
+      for (final accent in MornyeAccent.values.where(
+        (accent) => accent != MornyeAccent.red,
+      )) {
+        final scheme = MornyeTheme.build(
+          brightness,
+          accent: accent,
+        ).colorScheme;
+        expect(
+          contrast(scheme.primary, scheme.surface),
+          greaterThanOrEqualTo(4.5),
+          reason: '$accent on $brightness',
+        );
+        expect(
+          contrast(scheme.onPrimary, scheme.primary),
+          greaterThanOrEqualTo(4.5),
+          reason: 'Button text for $accent on $brightness',
+        );
+      }
+    }
+  });
+
   Future<void> openSettings(
     WidgetTester tester,
     SharedPreferences prefs,