It is great to hear Ali about your interest in Design. I learned all these when I was doing the designing course (unfortunately at that time the internet was not so reachable to us).
How to get started in design?
I would definitely advise joining a in-person course. The reason is you will learn a lot when you learn offline (in-person). As every designer has their own style and that can be explored when you are working with fellow designers and have a mentor. The first step you have already taken to learn the fundamentals of designs.
or you can join an interaction design forum - it is a very good website focusing on the UX and UI courses.
As for a front-end developer, I am assuming you ll always have a designer who ll take care of color theory and font-selection. All you need to take care of is to review the designs and make sure color-contrast is as per WCAG, there are not too many fonts, and spacing is consistent.
Thanks.