The DMZ theme is a good starting point since it’s already shipped with Ubuntu. You can find SVG files for all icons.
DMZ isn’t as sharp as something like OpenZone, so I’m really fond of that flat, sharp, no shadow look. If you try the proof-of-concept theme on my Github page, and switch between DMZ-White and the Communitheme you’ll see what I mean.
I’ll try to come up with something for the trobber/busy icon, and post it here.