Add cache-ability metadata for related theme settings

Problem/Motivation

Our theme settings changes currently depend on render cache clear.

Feedback from @Berdir:

That we have to do this implies that we are missing cacheability metadata. Since we rely on theme settings, we should add a cache tag to whatever elements this affects, e.g. the block.

That's probably a bit tricky and this isn't something that you set initially and then don't touch anymore.