The background color of RadSplitter Header is created with a background image which is repeated from left to right. If you want to change it, you should create your own image with the same dimensions - width 25 pixels, height 25 pixels.
Next step should be to overwrite the existing style in the header of your page to have the new image background visible:
Please, note that it is important to use the !important rule to overwrite the existing one. Also you should be careful which one of the existing skins you are modifying. In that case the modified skin is the Default one. If you choose to modify for example the Black skin, the CSS selector should be changed this way:
You can find attached the slidetitlecontainerbgr.gif, which is the background image that should be edited to change the background color of RadSplitter Header.
As this is a small change you could make it in the header of your page. If you like to have more visual changes it would be good to crate a Custom Skin for the Telerik Control you are using. You could find useful to read the following articles: Tutorial: Creating a Custom Skin for RadSplitter
and RadSplitter CSS Skin File Selectors
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