I am trying to change the banner color on a game I moderate (Disney Crossy Road). I want it to be black but when I just put a png of black there, nothing happens. Does it have to be a certain size? Thanks.

The banner only works when you don't have a background image.

Ok thanks. It thought the banner was the top bar that says "speedrun.com, games, streams, forums" etc. If you look at Minecraft, they have a different color top from their panel but also have a background image?

A banner is an image at the top of the screen that fades into the color of the background (see https://speedrun.com/baldnate or https://speedrun.com/tim_ for examples of banners). If you just want to change the color of the top panel, go to edit theme and change the "primary color" :

ok thanks.