I use my own userContent.css for Gmail.com for couple of months (different colors, hidden invitations box, etc.), but I have to say the gmailskins firefox extension (http://gmailskins.mozdev.org/) is really great! It adds itself to Gmail settings – you can change colors and positions of some elements. It displays preview image with your actual account.
Maybe the skins aren't exactly pretty ;), but the new release will focus on that.

I just installed this, but it doesn't seem to be compatible with Firefox 1.5.0.1? Whenn I install, restart Firefox, and visit Gmail to click "Settings" as it says, nothing happens.

I use Firefox 1.5.0.1 on Ubuntu Linux and it works...

Have you got this new gmail-chat future? I haven't yet. I guess It may breaks this extension. Try with https://.
Extension author will probably release new version in a few days.

You're right. I got the chat Gmail. That's probably it.

I meant "try with https" (I believe it doesn't display chat?). BTW – Did you see http://userscripts.org/tag/gmail ?

New version 0.8 – now works with accounts that have the new Gmail chat feature. Only the javascript has been updated (not the CSS). This means that the new interface elements are still default gmail blue. This will be addressed in an imminent release.

The next release will also include very nice mac os skin.

I hate to double post (pleas don't kill me) but does this violate Gmail TOS? I can't risk my account