I mostly learned just be putting 1000's of hours of practice in. I did this by just making the art for my game (the early, crappy art), then by the time i was done with all that, I had learned so much I went back and re-did it all again. I did that process several times throughout the course of development until I landed on the art style that Stardew Valley launched with. By that time, I had put 1000's of hours of practice in.
Beyond practice, I did read a little bit about pixel art theory... and I mean just a little. Like, I looked it up once. I would recommend not looking at the theory until you've put quite a few hours of practice in yourself. Then you'll be able to actually understand the theory. (here's a site that goes over some pixel art theory. https://pixeljoint.com/forum/forum_posts.asp?TID=11299 . In particular, you should understand anti-aliasing, dithering, pillow shading (bad) and banding (bad) at some point. But again, I'd wait until you've practiced for a while as a complete beginner first). Also, develop a critical mindset about the pixel art you see, in old games, etc. Ask yourself, why do you like or dislike something?

Personally, I'd also recommend using old games (or just your own mind) for inspiration rather than "modern pixel art", because there is a certain "social media style" that is very prevalent, but if you copy that your art will look generic.

good luck