Change image background in a 'Swell' stack

On my website ( I have a test page1 and using the Swell Stack from Doobox, I have added a couple of images to test the result.
I like the stack but, I want to have a plain coloured background (same as my site) when I click to expand the image in a lightbox Gary, from Doobox, suggests that I use a little ccs code to change the background colour.
Sorry, I have no real experience with ccs code, can somebody explain how I should proceed to change the background colour please?