[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.