When you have a dark GTK theme will result in some content being hard to read in Firefox. But, there’s fix for that, with the help of a firefox add-on and some CSS: Firefox dark GTK theme fix.
I’m using Eclipse for both Java and Python development at work, and I don’t really like the default UI and syntax theme.
A great resource for syntax themes I’ve been using for a while is Eclipse Color Themes. Since Eclipse 4, it is also possible to change the UI theme.
The Moonrise UI Theme looks really good!