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
skins/standard/, and put your directory in
skins/contrib/. Then, add your CSS to the appropriate files.
After you put the directory there, make sure to run
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 Preferences, on the General tab. If you would like to force a particular skin on all users, just select that skin in the Default Preferences in the Administration UI, and then uncheck "Enabled" on the preference, so users cannot change it.