Okay I'm still on the hunt for the elusive dark colour look-and-feel for NetBeans. I found this site that has great information about the NimROD L&F. This sets the widgets to a dark colour (or you could customise your own colours with NimROD).
Note that this is separate from the NetBeans colour themes which control editor colours. For that, I quite like Twilight and Aloha, although each needs tweaking to fix highlight colours for things such as breakpoints and step colours, or for highlighting code differences. Still, it's a good start.