I did this and it worked for me. I taught myself design after finishing a code-heavy computer science degree. I learned how to use Photoshop quite quickly along with some design theory but it took time to get really good.
I used to look at those sites that were popular a few years ago that list well designed websites, and take inspiration from one or several of them at a time by combining the elements to create a new design.
It helped that I was working for a company that wanted a new design created to a brief each week as it helped me to get better quite fast.
Reading a lot of design, UI and UX blogs to understand why things worked was crucial to my development too.