For the longest time, core skins had a big advantage: they could use the 'skinStyles' parameter of a ResourceLoader module definition, in core's Resources.php file, to provide different looks for various built-in functionality. Non-core skin creators could achieve the same by putting the styles in main skin styles, but this has two big disadvantages: the styles would be loaded even if not needed on given page, and one often had to use ugly hacks to "reset" core styles.

With [1] merged, non-core skins can finally do the same, and do it right, using the new $wgResourceModuleSkinStyles global (akin to $wgResourceModules):

// Module defined in core or some extension $wgResourceModules['bar'] = array( 'scripts' => 'resources/bar/bar.js', 'styles' => 'resources/bar/main.css', );

// Styles defined in the skin $wgResourceModuleSkinStyles['foo'] = array( 'bar' => 'skins/Foo/bar.css', );

Documentation is available at https://www.mediawiki.org/wiki/Manual:$wgResourceModuleSkinStyles.

I have already converted Vector to make use of this [2] and I am in the process of doing the same for Minerva [3][4] (the second patch still pending, reviews welcome) – MobileFrontend developers were forced to do some really bad things by the previous limitations ;)

Perhaps someone would be interested in doing this for MonoBook, too? It currently uses the "everything in main style file" approach.

Have fun!

[1] https://gerrit.wikimedia.org/r/#/c/141259/ [2] http://git.wikimedia.org/blob/mediawiki%2Fcore.git/1ebad72b0b45209ab04562716... [3] https://gerrit.wikimedia.org/r/#/c/149780/ [4] https://gerrit.wikimedia.org/r/#/c/150580/