I think you should ask yourself if and why you really want to get better at that. Why not work with a designer?
I'm all in favour of learning many technologies to get better at each. But it seems unlikely to me that your programming will improve much by learning design. It also seems unlikely that you'll get paid more.
If you do want to do it all yourself, my first bit of advice would be to go to a website with lots of designers, and ask there. It's a pretty separate skillset from programming.