Hello Markus Berchtold,
Thank you for using our services.
Please start by taking a look at the best practice for RadControls for ASP.NET AJAX
in Sitefinity. Then you can take a look at the following article to find about RadTabStrip CSS skin file selectors
. Then provide the skin in each theme you want to use it. Site TabStrip is a RadTabStrip wrapped in a user control and bound to our site map.
If you make any changes to the files in ~/Sitefinity/Usercontrols/
make sure you back them up before upgrade as the patch overwrites those files. Then just merge yours and the ones in the patch.
The controls located in ~/Sitefinity/Usercontrols/
do not external templates.
the Telerik team