refactor: Simplify themes and remove dead code

lib/widgets/lock_screen.dart

@@ -25,8 +25,8 @@ class LockScreenState extends State<LockScreen> {
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
-      theme: FluffyThemes.light(),
-      darkTheme: FluffyThemes.light(),
+      theme: FluffyThemes.buildTheme(Brightness.light),
+      darkTheme: FluffyThemes.buildTheme(Brightness.dark),
       localizationsDelegates: L10n.localizationsDelegates,
       supportedLocales: L10n.supportedLocales,
       home: Builder(

lib/widgets/matrix.dart

@@ -479,8 +479,8 @@ class MatrixState extends State<Matrix> with WidgetsBindingObserver {
       if (value != null && int.tryParse(value) != null) {
         AppConfig.colorSchemeSeed = Color(int.parse(value));
         AdaptiveTheme.of(context).setTheme(
-          light: FluffyThemes.light(),
-          dark: FluffyThemes.dark(),
+          light: FluffyThemes.buildTheme(Brightness.light),
+          dark: FluffyThemes.buildTheme(Brightness.dark),
         );
       }
     });