Shift + F1 will darken the appearance of Photoshop CS6’s interface by one step. Shift + F2 will brighten the interface by one step. Note: on a laptop, you might need to add the function key. Note: you can also change the appearance of the interface under Preferences > Interface > Color Theme.
In addition, you can Control -click (Mac) / Right Mouse -click (Win) in the grey area surrounding an image to change the color. This changes the interface behind your image, not the background color of your image.