Bugzilla allows you to have multiple skins. These are custom CSS and possibly also custom images for Bugzilla. To create a new custom skin, you have two choices:
Make a single CSS file, and put it in the
skins/contrib directory.
Make a directory that contains all the same CSS file
names as skins/standard/, and put
your directory in skins/contrib/.
After you put the file or the directory there, make sure to run checksetup.pl so that it can reset the file permissions correctly.
After you have installed the new skin, it will show up as an option in the user's General Preferences. If you would like to force a particular skin on all users, just select it in the Default Preferences and then uncheck "Enabled" on the preference.