DockColorize is a Cydia Tweak which will allow you to change the color of your Springboard Dock and/or your App Switcher. Not only are you able to change the color, but you can also SSH into your iDevice to set a custom image for your background. The Tweak has a few preinstalled colors which you enable by typing out the file name in the Settings. This is not the prettiest way of doing it, but if you're going to be adding custom images then it's possibly the best solution. DockColorize can be found for FREE under the Big Boss Repo.