To install a theme, download one and copy the files to the "apps" folder, just like an homebrew application.
As soon you load it from the Homebrew Channel, the theme will be installed.
Note: You must have the Homebrew Channel 1.0.7 or higher in order to use themes. Update your Homebrew Channel with the HackMii Installer.
It should be:
and then in here you should have a .zip file , a .xml and a .png.