One of the first things I wanted to fix on GNOME when I got involved in the project, was the theme handling.
If you double click on a theme package (tar.gz file), you find yourself looking to a file-roller window, and you have to figure out that you need to unpack it on $HOME/.themes if it's a Metacity/Gtk+ theme or $HOME/.icons for Icon themes. Quite hard despite there is a drag and drop tip on http://art.gnome.org/.
I started wondering how hard would be to create a new MIME type for theme packages, first I need a way to install packages through the command line, first I filed this bug (#354581) which was fixed by thos within a few weeks (yes, thos rocks ;-) now, you can do this (on SVN trunk):
arc@dopamine ~ $ gnome-theme-manager -i GTK2-Theme.tar.gz
The next step was the MIME type definition, and the .desktop file to associate the MIME type and the installation command. With some help from the guys from #control-center at irc.gnome.org during the last patch party, I filed a bug (#393697) and wrote a patch for control center to fix it. This is probably the hardest patch that I've made for GNOME until the moment so I'm very happy. I wouldn't be able to write it without the help from thos, jpetersen and rodrigo. Thank you guys :-)
You can get more information about MIME Types in gnome on the GNOME Sysadmin Guide.
The patch is supposed to be commited tomorrow before the feature freeze, I'll keep my fingers crossed, if it's not commited tomorrow, this feature would need to wait until the next release.