I'm looking for an extension what will apply a specific stylesheets to specific websites. For example
googlestyle.css would go with Google.com and
superuserstyle.css would go with superuser.com.
Stylish lets you easily manage user styles. Add, delete, enable, disable, and organize with a few clicks of a mouse, no code to edit, no obscure configuration to find. Stylish's companion website, userstyles.org, hosts tens of thousands of user styles made by other Stylish users that you can try.
(Despite the above, it's just as easy to use your own stylesheets for any website).