I think that the following picture should clarify things better:

http://www.betaarchive.co.uk/imageupload/1261105951.or.536400.png

I'm really talking about the tab content which has a slight gradient. It certainly seems that in Windows 7 default theme, it has only one color, but in Windows XP, it is a set of colors due to the gradient. Which means that in the future, other theme might appear with various possibilities for the background color.

As for PNG, make sure to load them via an EV_PIXEL_BUFFER which is better at handling images (it can load JPEG for example) and use an EV_DRAWING_AREA to draw them.