Improve dark theme

lib/helpers/preferences_helper.dart

@@ -71,3 +71,10 @@ class PreferencesHelper {
     return v == null ? alternative : v;
   }
 }
+
+PreferencesHelper preferences() {
+  if (PreferencesHelper._instance == null)
+    throw Exception("Try to get preference before their initialization!");
+
+  return PreferencesHelper._instance;
+}