AFAICT it's Firefox's CSS+HTML engine running the user chrome. You can even open the firefox web inspector on it (open web inspector -> settings -> enable brows

AFAICT it's Firefox's CSS+HTML engine running the user chrome. You can even open the firefox web inspector on it (open web inspector -> settings -> enable browser chrome and add-on debugging toolboxes) which lets you fiddle with the chrome DOM and modify CSS that way.