Actually, not as much as you might think. It's much more than just changing some images and colors. Very often, you find changes to a skin will cause errors in an application. It could be something as simple as an object's width, or a major issue like a redraw glitch in the coding if togglable items are overlapping.

When creating UIs you need to work closely with your engineers so you have an idea of what the architechture you are building with is designed to do, as well as how far you can bend and push it to enhance the client designs.