Which framework for a graphic designer?

Hi,

I am a designer, but not much of a coder. I can hack together php & css, but am not well versed to do it from scratch.

In the past, I have altered available themes with different graphics to get the kind of look I was going for. This is obviously bad practice.

So, I want to start using frameworks to design child themes for me to use. Which framework would be the best for me. Obviously, over time, I will learn more php/css/wordpress codex so I want something that I can delve into immediately but will continue to be useful as I develop my coding skills.

Thoughts?