Update Dec 2012:
I have just started playing with the Stylish plugin to Mozilla Fox. It allows you to override a site's stylesheed on a per-site basis. There seems to be a good number of user-defined themes at http://userstyles.org/
I am delighted to find a "Nightshade" theme, and it's pretty close!
|Stylish plugin to Mozilla Firefox, "gmail-b2b" + "Nightshade" theme|
|Stylish plugin to Mozilla Firefox, "gmail-b2b" + "Nightshade" theme, compose view|
I think we'd have a winner!