This is by far the biggest deep dive I’ve seen on CSS Variables posted to the web and it’s merely Chapter One of complete e-book on the topic.
Truth is, I’m still on the thick of reading through this myself, but had to stop somewhere in the middle to write this up and share it because it’s just that gosh-darned useful. For example, the post goes into great detail on three specific use cases for CSS Variables and breaks the code down to give a better understanding of what it does, in true tutorial fashion.
Scoping, inheritance, resolving multiple declarations, little gotchas—there’s plenty in here for beginners and advanced developers alike.
Frontend Masters has an in-depth course on Advanced CSS Layouts that digs into the big stuff like grid and flexbox, and then more like custom properties and calc(), using them for advanced layouts and practical solutions.