It’s all a learning curve and the best way to learn is to jump in and start coding but expect to fail often.
These days CSS is so vast that it’s a lifelong pursuit to know it all and it’s constantly changing anyway. Just Flex and Grid could take a year or more to master and indeed some people never do master them but are still able to use it if they stick to what they know.
Of course the basic fundamentals should be learned first but once past that hurdle then anything is game 
We all learn from our mistakes.