A change to CSS should not affect the behaviour of a page, only it's appearance, so I'm not sure that Selenium would be much help for this.
I'm going to take a guess that you are trying to avoid problems such as elements being misplaced on the page so that they are not readable. If so, you would probably need some kind of OCR-based tool, but I don't know one off-hand to suggest.
However, it may be to better to invest your effort in preventing this kind of problem in the first place. If your layout is easily broken, maybe you need to refactor your CSS to something simpler.
