User preference For site color css file

Hi,
I have a website template. It have color css files.
Black.css (default)
Blue.css
Red.css
Green.css
Users have 4 options for visual. User can select a option when it was, continue to use website.
How must to use it. Cookies or something?

Can you give me a example about that? I guess I have to use a cookie in _layout page.