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+There are three directories here, standard/, custom/, and contrib/.
+
+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.