Add themes and drawing field for kanji

lib/service/theme_bloc.dart

@@ -0,0 +1,26 @@
+import 'package:flutter_bloc/flutter_bloc.dart';
+import 'package:flutter/services.dart';
+import 'package:meta/meta.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+export 'package:flutter_bloc/flutter_bloc.dart';
+
+@immutable
+class SetTheme {
+  final bool isDark;
+  const SetTheme(this.isDark);
+}
+
+class ThemeBloc extends Bloc<SetTheme, Brightness> {
+  final SharedPreferences prefs;
+
+  ThemeBloc({
+    required this.prefs,
+    required Brightness init,
+  }) : super(init) {
+    on<SetTheme>((event, emit) {
+      prefs.setBool('darkTheme', event.isDark);
+      emit(event.isDark ? Brightness.dark : Brightness.light);
+    });
+  }
+}