The CSS Cascade is a beautiful website that explains how inheritance works in CSS. This concept is absolutely essential to understand how CSS works. It’s also important to know the cascade works especially if you are trying to override something in the Drupal theme.
Flexbox Zombies game is a great way to learn flexbox. It’s entertaining and has practical examples.
Josh Comeau has written a a great interactive introduction to flexbox. There’s kebabs, Clippy, and cocktail wieners. Very thorough and delightful.
Complete Guide to Flexbox from CSS Tricks is great for a reference after you’ve learned it. You can read their collapsed sections at the top about the Background and Basics and terminology. But I’ve found that those concepts are more approachable learning from videos and practicing on your own.
The Best Thing to Do
You can watch all the videos and read a hundred Stack Overflow threads, but the best thing to do is practice. Get a CodePen account and start creating something! Practice! Practice! Practice!