To change the background-color of non-selected tabs when RadTabStrip is in Lightweight render mode, you need to add the following CSS to your page:
html .RadTabStrip .rtsLevel
Please note that this style doesn't include hover state of the tabs.
Attached you may find a sample project. I also took a screen-shot
showing how the RadTabStrip is displayed at my side.
Hope this will be helpful.
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