This is done by using a custom style sheet. The basic principle is to design another set of style and with every styles add "body#gmail-google-com" as prefix (i.e. the ancestor element). Install the firefox extension URL id , copy the CSS to your chrome directory and rename it. Then Firefox would apply to style in the file to the specific web site the id mentioned. This is quite an interesting way to skin your web content as long as they are using CSS to layout and style almost everything.
Of course, you have to use Mozilla / Firefox.
Related Link: Skinning Gmail with a Custom Stylesheet