CobbleDark is a customized version of Lokheed's GTK+ Cobble theme. It has been stripped of the original orange and blue colors and reworked as a gray theme, including black panels, menus, and progress bars.

Wallpaper by gemini-kriket, found @ http://www.deviantart.com/deviation/56392356/
Lockheed's Homepage, http://lokheed.deviantart.com/

Warning:
This theme will not play nice with Firefox by default. You will need to edit the "userChrome.css" file located in the "chrome" folder of the default Firefox profile directory.

For example, "/home/matt/.mozilla/firefox/gbdlpb0x.default/chrome/userChrome.css".
Append the following lines to that file (if it does not exist create it):

menu { color: white !important }
menupopup > * { color: black !important }

Save and restart the browser. The menu should now appear correctly. The same will work for Thunderbird.