Bugfix to avoid that ThemeData doesn't get changed correctly when darkTheme is s…

…et in GetMaterialApp and Theme should be changed to a ThemeData with Brightness.dark.

@@ -37,11 +37,15 @@ class GetMaterialController extends GetxController {
   Map<int, GlobalKey<NavigatorState>> keys = {};
 
   void setTheme(ThemeData value) {
+    if (darkTheme == null) {
+      theme = value;
+    } else {
       if (value.brightness == Brightness.light) {
         theme = value;
       } else {
         darkTheme = value;
       }
+    }
     update();
   }