Timeless doesn't actually use these from what I see. Timeless copied and pasted across the deprecated stylesheet. I'm not sure what plans it has for the future but it doens't block completing this work.

Change 774521 had a related patch set uploaded (by Jdlrobson; author: Jdlrobson):

[mediawiki/skins/Timeless@master] Timeless should use message box styles from core

https://gerrit.wikimedia.org/r/774521

I stand corrected. For some reason Timeless is disabling the interface styles:

Patch submitted.

In T304768#7811995, @Jdlrobson wrote:

For some reason Timeless is disabling the interface styles:

Are you sure it’s Timeless, and not, for example, a browser extension? I see the yellow warning box on https://en.wikipedia.org/wiki/Special:UserLogin?useskin=timeless&safemode=1:

@Tacsipacsi did you remove the deprecated warningbox class e.g. $('.warningbox').removeClass('warningbox')?

Timeless is shipping its own styles here by using an outdated specification here:
https://gerrit.wikimedia.org/g/mediawiki/skins/Timeless/+/9f8e03b66626c203573b77447bafa5240baa5b1b/resources/libraries/shared.css#369

and its opting out of the Wikimedia-maintained CSS here:
https://gerrit.wikimedia.org/g/mediawiki/skins/Timeless/+/9f8e03b66626c203573b77447bafa5240baa5b1b/skin.json#69

In T304768#7812869, @Jdlrobson wrote:

@Tacsipacsi did you remove the deprecated warningbox class e.g. $('.warningbox').removeClass('warningbox')?

No, you didn’t say explicitly that I need to, and I wasn’t clear for me that you meant that implicitly. If I remove the class, it gets unstyled for me as well.

Change 774521 merged by jenkins-bot:

[mediawiki/skins/Timeless@master] Timeless should use message box styles from core

https://gerrit.wikimedia.org/r/774521