Soon there will be TemplateStyles (T133410), and svwiki will be the first production wiki where it is deployed (T176082). When that happens, I intend to move styles from the MediaWiki namespace to individual templates.
If the performance team wants, I could announce here on Phabricator when I add a <templatestyles> tag in a template, and when I remove CSS from the MediaWiki namespace, if that would help you determine whether there is any improvement or degradation in performance. The candidates for this migration are listed at https://sv.wikipedia.org/wiki/MediaWiki-diskussion:Gadgets-definition#Kandidater so that you can get a sense of the amount of CSS we're talking about.
If you're not interested, you can just decline this task.