Beginner question

Hi. Please forgive me, I’m very new to this.

I’m just working on a personal blog site (and setting them up for friends). I’ve got them working pretty well and can use themes that others create.

I’d like to get into some more in-depth creation and modification of themes (right now I’m just muddling through the code and replacing paths and images where I can find them).

Does anyone have a good recommendation for an overview of creating themes and css? My background for web design is VERY old school (like early 90’s HTML), but I’m pretty compotent with the logic.