Polished layout and icons

lib/screen/pages/more/gradeAverages/gradeAverage.dart

@@ -53,7 +53,7 @@ class _GradeAverageState extends State<GradeAverage> {
           PopupMenuButton<bool>(
             enableFeedback: true,
             initialValue: gradeSystem,
-            icon: const Icon(Icons.read_more),
+            icon: const Icon(Icons.more_horiz),
             itemBuilder: (context) => [true, false].map((e) => PopupMenuItem<bool>(
               value: e,
               enabled: e != gradeSystem,