Add dark theme

lib/ui/routes/app_settings_route.dart

@@ -1,3 +1,4 @@
+import 'package:comunic/helpers/preferences_helper.dart';
 import 'package:comunic/utils/intl_utils.dart';
 import 'package:flutter/material.dart';
 
@@ -23,9 +24,81 @@ class _AppSettingsBody extends StatefulWidget {
 }
 
 class __AppSettingsBodyState extends State<_AppSettingsBody> {
+  PreferencesHelper _preferencesHelper;
+
+  @override
+  void initState() {
+    super.initState();
+    _load();
+  }
+
+  void _load() async {
+    final prefs = await PreferencesHelper.getInstance();
+    setState(() {
+      _preferencesHelper = prefs;
+    });
+  }
+
+  void _needRefresh() {
+    setState(() {});
+  }
+
   @override
   Widget build(BuildContext context) {
-    return ListView(children: <Widget>[],);
+    if (_preferencesHelper == null)
+      return Center(child: CircularProgressIndicator());
+
+    return Padding(
+      padding: const EdgeInsets.all(8.0),
+      child: ListView(
+        children: <Widget>[
+          // Dark theme
+          _CheckBoxPreference(
+            preferenceKey: PreferencesKeyList.ENABLE_DARK_THEME,
+            preferencesHelper: _preferencesHelper,
+            onUpdate: _needRefresh,
+            title: tr("Enable dark theme"),
+            description:
+                tr("You will need to restart the application to apply changes"),
+          ),
+        ],
+      ),
+    );
   }
 }
 
+class _CheckBoxPreference extends StatelessWidget {
+  final PreferencesKeyList preferenceKey;
+  final PreferencesHelper preferencesHelper;
+  final VoidCallback onUpdate;
+  final String title;
+  final String description;
+
+  const _CheckBoxPreference({
+    Key key,
+    @required this.preferenceKey,
+    @required this.preferencesHelper,
+    @required this.onUpdate,
+    @required this.title,
+    @required this.description,
+  })  : assert(preferencesHelper != null),
+        assert(preferencesHelper != null),
+        assert(onUpdate != null),
+        assert(title != null),
+        assert(description != null),
+        super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return SwitchListTile(
+      value: preferencesHelper.getBool(preferenceKey),
+      onChanged: (b) async {
+        await preferencesHelper.setBool(preferenceKey, b);
+        onUpdate();
+        return true;
+      },
+      title: Text(title),
+      subtitle: Text(description),
+    );
+  }
+}