Theming API and CSS File Themes are both used in the same page. In v33 we released the Theming API as the new default method of styling the grid. See the migration docs https://www.ag-grid.com/react-data-grid/theming-migration/. Because no value was provided to the `theme` grid option it defaulted to themeQuartz. But the file (ag-grid.css) is also included and will cause styling issues. Either pass the string "legacy" to the theme grid option to use v32 style themes, or remove ag-grid.css from the page to use Theming API.
To show this error text in the dev console, import the ValidationModule.