Fixed theme's list not auto-updating on upload

no issue

- the ember data live record array needs to live on a tracked property rather than a getter for the template to auto-update

ghost/admin/app/controllers/settings/design/change-theme.js

@@ -8,6 +8,7 @@ export default class ChangeThemeController extends Controller {
     @service themeManagement;
 
     @tracked showAdvanced = false;
+    @tracked themes = this.store.peekAll('theme');
 
     marketplaceThemes = [{
         name: 'Edition',
@@ -43,10 +44,6 @@ export default class ChangeThemeController extends Controller {
         shortImage: 'assets/img/themes/Ease-cut.jpg'
     }]
 
-    get themes() {
-        return this.store.peekAll('theme');
-    }
-
     @action
     toggleAdvanced() {
         this.showAdvanced = !this.showAdvanced;