While I am still doing final upgrades and additions for the PostX Gnu/Linux 0.3 repository – and final checkups as well – here is a quick tip on changing themes on pretty much all current Linux systems.
If you use Openboox, Fluxbox, Jwm or anything else lightweight install lxappearance: sudo apt-get install lxappearance
If you install modern Gtk3 themes then do not be amazed if the new theme might not show up as expected. Gtk3 themes are sometimes moving targets and as such require new theme engines. Occasionally, the previous leads to the scenario where theme x does not find the requirements it needs to function correctly. Gtk2 themes however usually work as expected since the theme engines required are older – and thus usually found within pretty much every current Linux distribution. There is no clear way to say if a theme x will work on system y. The best practice is to try it out and see. If the theme fails because it needs a newer theme engine then one way to patch the issue is to compile the engine from source. See this post if you want to compile a Debian package from source: http://www.techtimejourney.net/how-to-build-a-debian-package-case-example-with-midori-0-50/
You can also use a tool called gnome-tweak-tool if you wish. The tool itself is Gtk3 related and does not serve user well under Gtk2. As far as I can tell Lxappearance is the best tool for simple theme changing – unless you do not want to do it manually by yourself.