if you really want to get into pixel art things, you should look at the difference between 8-bit graphics and 16-bit graphics. right now your tileset is 8-bit, quite reminiscent of some of the older pokemon games for instance. some artists prefer a 16-bit aesthetic like the JRPGs on the SNES--that one typically requires a lot of work to get it to look great, since you can't have empty swaths of a color like you have right now for instance.
it's your game so do what you like, but imo you should keep the 8-bit graphics for the overworld and make 8-bit sprites for the characters and items and things to match it. it's much less work and less frustrating for a first time game dev
of course you don't have to do actual 8-bit palettes and whatnot. just use the style. unless again you wanna get really into the nitty gritty of the pixel art. up to you