Selecting an artstyle...

Is there a list of common artstyles? I have waited until my gameplay is pretty much ready, but everything looks wrong. When it comes to polishing are there some guidelines to go by to make everything look right?

 

Yes: hire a professional artist.

Seriously. These guys are amazing. You may or may not be able to get a half-decent look on your own, but a good artist will always make it shine.

 

Yeah, there isn't really a list since each artist (or art director) sets the rules for individual projects. Theres a lot to consider, and in a lot of cases, the artists will need to make a judgement call about how to make something specific fit nicely into the rest of the world. They use the art style, and any style guides as their reference point to stay grounded and to keep the world consistent and believable.

As an answer to your question, if you want somebody else's art style, just grab screenshots from their game and learn how to replicate their work. I don't recommend this at all, but existing games are as close to a list of art styles that I can think of. Be aware though, if you don't have any since for what makes a good art style, you could end up using a game with tribe art direction as your muse