Bugzilla supports skins - ways of changing the look of the UI without altering its underlying structure. It ships with two - "Classic" and "Dusk". You can find some more listed on the wiki, and there are a couple more which are part of bugzilla.mozilla.org. However, in each case you may need to check that the skin supports the version of Bugzilla you have.
To create a new custom skin, make a directory that contains all the same CSS file names as :file:`skins/standard/`, and put your directory in :file:`skins/contrib/`. Then, add your CSS to the appropriate files.
After you put the directory there, make sure to run :file:`checksetup.pl` so that it can set the file permissions correctly.
After you have installed the new skin, it will show up as an option in the user's :guilabel:`Preferences`, on the :guilabel:`General` tab. If you would like to force a particular skin on all users, just select that skin in the :guilabel:`Default Preferences` in the :guilabel:`Administration` UI, and then uncheck "Enabled" on the preference, so users cannot change it.