There are four directories here, standard/, custom/, contrib/, and assets/. standard/ holds the standard stylesheets. These are used no matter what skin the user selects. If the user selects the "Classic" skin, then *only* the standard/ stylesheets are used. contrib/ holds "skins" that the user can select in their preferences. skins are in directories, and they contain files with the same names as the files in skins/standard/. Simply putting a new directory into the contrib/ directory adds a new skin as an option in users' preferences. custom/ allows you to locally override the standard/ and contrib/ CSS. If you put files into the custom/ directory with the same names as the CSS files in skins/standard/, you can override the standard/ and contrib/ CSS. For example, if you want to override some CSS in skins/standard/global.css, then you should create a file called "global.css" in custom/ and put some CSS in it. The CSS you put into files in custom/ will be used *in addition* to the CSS in skins/standard/ or the CSS in skins/contrib/. It will apply to every skin. assets/ holds the minified and concatenated files which are created by checksetup.pl and Bugzilla::Template. Do not edit the files in this directory.