I asked the very same question on vb.com.
Goto your styles page, where you enter your colors, edit header etc. In the catagory strip color box, where its #93939 hex for the color, add something like this:
#DFDFDF" background="{imagesfolder}/yourimage.gif
Thats how I have it on my test forum (working out the gradients some more then I'll put it on my real forum).
I simply made like a 25pixel high, 1 pixel wide image in Photoshop, and chose a gradient. Save it as .gif, upload it, and enter the code and it should work.