All you have to do is read up on using Facebook Connect. Here's the article on the like button and tying it in with your website, Like Button - Facebook Developers .
You'd follow what they said and then style the code they give you using CSS to make it blend in better with your website.
I don't think it's possible with just CSS. The Facebook Like button loads in an iframe. Browser security policies prevent you from altering the contents of an iframe if it comes from a third party.
Some people in this thread are saying this is illegal or against Facebook T0S. Well it's obviously not illegal and I haven't been able to find it in their T0S/T&C. However, I did find this under their guidelines for usage of the Like button:
"While you may scale the size to suit your needs, you may not modify the Like button in any other way (such as by changing the design)."