That... is the Vector design. Vector isn't modern. That's kind of the defining thing about Vector.

But if someone wants to do something truly useful with this, there are in fact quite a few different components scattered around core (mediawiki.skinning.interface and various content modules), extensions, and onwiki content, that either are or should be using styles consistent with the catlist, toc, wikitables, etc. Some try by mimicking the same styles (making it very difficult to make things consistent in any skins using different styles), others reuse classes really not meant for wider use, many just don't bother at all. Figuring out what all those are and establishing a more specific set of classes to style them all such that new extensions and components, or even specific templates on wiki can also consistently reuse them, would be incredibly helpful for both skinning in general as well as simplifying the design process on new extensions and content alike.

It occurs to me that what I'm describing is actually a somewhat major thing, so I've moved the bulk of it to a new task: T220722

@Isarra Vector seems modern to me (in comparison with Cologne Blue, Modern or MonoBook skins). What I want to say is that I don't think categorybox and TOC design fits into the current Vector design.

For such a task, not only for helping with current communication about design preferences, versus inconsistencies, versus what is being referred to as “modern” or “Vector design” or “90's” (!), but also for longer term understanding, it would be helpful to have screenshots and a bit more explanation for those terms.