I was going to recommend Jonas Schmedtmann's Advanced CSS course but you already did. It's amazing!
He also has this course that you can take before the advanced one if needed... I haven't taken it but I'm tempted to. udemy.com/design-and-develop-a-kil...
That's a good one too. It really goes to show that being a 'full stack' developer tends to leave a tremendous deficit in other areas of expertise.
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