Some skins jumble their CSS together into one huge file. Ideally, this should be separated out and only loaded when needed.
A good example is Refreshed's main.css. Some CSS is specific to the Echo extension, while others are directly specific to viewing action=history, etc. See https://gerrit.wikimedia.org/r/#/c/347170/ for more information on how to create a similar patch, along with https://github.com/wikimedia/mediawiki/blob/master/resources/Resources.php to figure out how more easily which modules should be referenced in skin.json.