Can anyone refer me to a site that uses dark backgrounds, etc with these extensions implemented?
Most other extensions I have seen are not at all geared for dark themes as they look best on a white background.
What level of customisation is available with the colours?
The amount of customization for most of the social widgets (Like, Comment, Fan, etc) is dictated by Facebook. The modules are all loaded from Facebook, and each usually has a "colorscheme" option for Light or Dark. However, it can be limiting, we'll be honest. In the past, Facebook has actually allowed more control through mechanisms like loading your own CSS sheet for each module. Over the last 6 months, they've slowly stripped this ability from each module as it was abused to basically make the Like button (for instance) look nothing like a Like button, which hurt Facebook's brand and could confuse users (that's their explanation anyways).
For an example of what the "Dark" scheme looks like, below is one example site that I was recently helping a user with. The like button is in the top-right corner:
Of course, the social modules are only about half of what JFBConnect has. The main, most prominent feature, is the single-sign on and user creation functionality.
As always, we have a 30-day money back guarantee. If you aren't satisfied, it's not a problem.
Hope this helps, and good luck!