Being a space program fan, I use the Firefox - 'NASA Night Launch' theme. I noticed this morning an add-on, the 'Dark Gmail2 Nasa' Extension, that compliments it. It basically extends the NASA theme down into the GMail web interface. I dig it. While I prefer to do my GMailing through Thunderbird, this will be a cool interface when I am out and about and using my thumb drive's U3 version of Firefox.
Check it out via your Firefox's Themes and Add-ons.