ThemeManager: Add support for checkboxes

Author: skybird23333

lib/util/theme_manager.dart

@@ -69,6 +69,10 @@ class ThemeManager {
       iconTheme: IconThemeData(
           color: primaryColor
       ),
+      checkboxTheme: CheckboxThemeData(
+        checkColor: MaterialStateProperty.all(backgroundColor),
+        fillColor: MaterialStateProperty.all(primaryColor)
+      )
     );
     return theme;
   }