We move the Bugzilla stylesheets, which currently live in css/, to skins/standard/, and we call them the standard skin. We style all Bugzilla pages with skins/standard/global.css, and we style pages needing special styling with page-specific files (f.e. skins/standard/buglist.css). Images and related files go into a page-specific subdirectory (f.e. skins/standard/buglist/image.png).
[We're moving from css/ to skins/ because modern style often includes images, and we need to store them somewhere, and it'll be easier to install a third-party skin into a single directory, and "css" is misleading if we're storing images there too.]
Custom styles live in skins/custom/. Standard stylesheets tell customizers to hack equivalent files in skins/custom/ instead. They also mention that customizers can put all rules, including page-specific rules, into skins/custom/global.css for file management simplicity, but they note the selector complexity and performance cost of this approach.
checksetup.pl creates skins/custom/ and a set of subdirectories/empty stylesheets equivalent to the files in skins/standard/. skins/.cvsignore tells CVS to ignore skins/custom/.
[skins/custom/ shouldn't be managed by CVS so it doesn't complicate the output of "cvs diff" and installations can store it in local CVS. It should be global (i.e. affect all skins) so installations can use it to make changes that apply regardless of the user's choice of skin.]
Installation admins can install third-party skins by adding them to the skins/ directory and then adding their name to the "skins" parameter. Skins can be either single CSS files (skins/<name>.css) or directories of files (skins/<name>/) with the same directory/file structure as the default skin. Multiple skins show up as alternate stylesheets, and users can switch to them via their browser's standard mechanism for switching between alternate stylesheets.
Bugzilla uses the standard skin by default. Admins can make Bugzilla use a third-party skin by default via the "default_skin" parameter. Multiple skins appear to the user as alternate stylesheets. Bugzilla populates the list of alternate stylesheets with the contents of the "skins" parameter, excluding the current default skin.
header.html.tmpl generates persistent stylesheet tags for customizations, preferred stylesheet tags for the default skin, and alternate stylesheet tags for other skins. Pages thus take on the default style, as modified by custom styles, until the user selects an alternate style, whereupon they take on the alternate style, as modified by custom styles.
More on persistent, preferred, and alternate stylesheets:
We classify rather than identify pages by installation (i.e. we move the installation-specific value of body tags' "id" attributes to their "class" attributes). We additionally classify pages by CGI. We give pages unique IDs where appropriate. For example, a show_bug.cgi page may be identified by the bug number it displays.
Implementation of this proposal will happen in stages. In the first stage, we move existing files to their new locations, make checksetup.pl create the custom directory/file hierarchy, make header.html.tmpl include custom stylesheets, and classify pages by installation and CGI.