Ask HN: How can I master CSS?

CSS seems simple when I read about it, but I just can't master it. For simple layouts, I have no problems. Start doing very complex layouts and throw Bootstrap and/or other libraries into the mix, and I'm lost.

Mentally, I don't feel lost, but rules don't behave the way I expect them to. It's probably not true but my impression of CSS is that it's incredibly inconsistent and difficult to work with. As projects grow these issues compound.

My backend programming skills are decent but I want to push my frontend skills to the next level -- which means I really need to master CSS.

What can you suggest that will help me gain the confidence I need?