White text wanted on dark blue desktop

I am setting up a Squeeze+Trinity box to demonstrate to someone with very poor sight. He is also a newbie, which means that everything must be fully labeled. I want to be able to show a light background to the desktop with dark/black text under the (enlarged) icons and a dark background with white text. This is something I have done in the past with KDE 3.x, but can't remember how. It is desirable, because many partially sighted people find white on black easier to read than black on white. This chap may not, hence my wanting to show him both. Help, please. Thanks, Lisi