That's a foreground image.

try style="background-image:url('newRightTab.gif');"

In my experience, I've had much more joy using Dojo tabs instead of the XPages inbuilt ones.

 

Firstly, validation does not work across tabs (until you have loaded them for the first time) and they are rather slow and clunky when compared to the dojo ones. 

 

Here is an example of how smooth they can be without any page reloads (obviously, the initial page load is slightly slower).  I've played about with programatically selecting a tab and the 'closable' ones and they are really good.

 

 http://dojotoolkit.org/reference-guide/dijit/layout/TabContainer.html

 

I will look at the alternative dojo tabs because changing the actual background colour of the tab seems impossible.

 

Thanks