[h4l]

I would probably start by opening the browser dev tools on web pages and use inspect element to see how an element in the page has been made to look the way it is. You can edit properties to see what effect they have when changed.

The domain of styling html is so large that there's no chance of learning everything in theory then applying it, I think it's best to learn by doing.

[gingerlime]

I’ve been doing that for over a decade, without making tangible progress.

[h4l]

You've probably accumulated a lot of relevant html knowledge which will help you.

I presume you have some css-related projects/tasks in mind that you'd like to be able to tackle yourself. I would pick one of those and jump in at the deep end. Although a small goal like restyling something that already works is probably better than starting from scratch.

For a completely abstract task, you could try developing a stylesheet for markdown html output.

Vanilla CSS is good enough (and browser support consistent enough) now that you don't need to use any compilers/frameworks to get started.

If you find yourself struggling for design inspiration, try to re-produce an existing design without seeing how they did it.