Thank you for writing.
Changing those colors is a very easy operation with the provided EditUI menu for our controls. When you select the RadTabStrip simply click on the smart tag and edit those properties that you want. Have in your mind that the settings that you want to change are located in the FillPrimitive. I am attaching a screen shot to guide you on making those changes in your case. This can be done with most of our controls.
About the Theme creation you can export the build-in themes and then load one of them and modify it by your discretion. Take a look at the following links in our help for more info:
I hope you find this information helpful.
the Telerik team
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