>I understand CSS, what the properties do, individually. But I lack the creativity and design skills required to come up with half of what is provided by those frameworks. I could do it if I had infinite time and patience, but I don't.

You know the syntax but you don't know how to think in CSS. Learn how to create layouts. Learn flexbox and grid. Watch a few YT videos to and try to design something yourself.

For example take a website like Facebook and try to design the log-in form like that. That's how you learn. No one is born with perfect design skills and it doesn't take a lot of time to learn how to make designs so awesome you won't believe you did them yourself in pure CSS.