It's all shaders and I'd like to say a somewhat decent art direction. I'm no pixel artist so I won't try to do that but I know basic color theory so I trust in my ability to judge what's looking appealing at least in that regard. But mostly just shaders. Oh, and maybe you didn't realize but my shaders are also not working properly - I just worked on them until the mistakes are less noticeable XD. Seriously shaders are tricky at first don't let that keep you from learning them. Xor has some of the best game maker shader introduction tutorials out there https://xorshaders.weebly.com/ . He's just great! And I also liked this video series a lot
Oh and just look at other people's code. If I recall correctly there should be some free shaders on the gm marketplace to download! Hope that helps!