use builtin modern css as basis, because despite its ugliness, it fixes three...

use builtin modern css as basis, because despite its ugliness, it fixes three annoying issues with the builtin default v2 theme: it ensures indents work for dynamic matrix questions, it doesnt display a double hover popup when a validation fails in a matrix question field, and it fixes the layout of multiple textboxes questions

survey-frontend/src/index.tsx

@@ -4,7 +4,7 @@ import { createRoot } from 'react-dom/client';
 
 //import 'bootstrap/dist/css/bootstrap.min.css';
 
-import 'survey-core/defaultV2.min.css';
+import 'survey-core/modern.min.css';
 import surveyJson from './survey_model.json';
 import { Model } from 'survey-core';
 import { Survey } from 'survey-react-ui';