Do you have a link to some example usage?

It's all over the place. Illustrative search.

CodeSearch says its in core: resources/src/mediawiki.skinning/content.css.

This probably applies to all of my skins, bar Anisa, which may actually be the only one using regular monobook interface styles. >.>

Note that this allows users to consistently apply skin-styles to content, and is thus actually somewhat important. In this case, it should be as simple as applying the .box and similar mixins, or adding them to the same list as in content.css.

Change 683777 had a related patch set uploaded (by Isarra; author: Isarra):

[mediawiki/skins/Timeless@master] Support mw 1.36/1.37: Disable all skin features except for a couple in favour of mw 1.34 shared.css to avoid weird inconsistent visual styles

https://gerrit.wikimedia.org/r/683777

Welp, looks like this was resolved in core by toccolours being set in the toc feature, which means all my skins are going to have to override that back to something that matches too... :P

Change 683777 merged by jenkins-bot:

[mediawiki/skins/Timeless@master] Support mw 1.36/1.37: Disable all skin features except for a couple in favour of mw 1.34 shared.css to avoid weird inconsistent visual styles

https://gerrit.wikimedia.org/r/683777