Hi Markus Berchtold,
Thank you for your feedback. You can do the basic layout in Forms editor and then fine tune the appearance with CSS in the theme applied to the page where you have added the form. You can set CSS classes to the wrapper tags of the widgets and use them in the theme to achieve the expected design.
If you are experiencing any difficulties styling your form you can send your theme, form widget settings and page layout settings and will do my best to help you.
All the best,
the Telerik team
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