Hello Tal Bazadka,
From what I understand, you want to change the color of the selected tab in a tabbed document. If this is so, then you should change it through the Visual Style Builder. General help for the Visual Style Builder and themes can be found here
. I have attached a screenshot showing how you can select the tab item in the Visual Style Builder.
If you need any further help, do not hesitate to contact me.
I hope this helps. If it did not, then please send us some more details. This will let us provide you with more appropriate answer.
the Telerik team