I'm trying to get rid of the horizontal scroll bar at the bottom of my game, but the only way I can do this is to reduce my game width. However, when I do this, I end up with white margins either side of my game area. Is there any way to set the colour of the game borders?

Not sure if this is helpful, but before I followed suggestions that worked, I used a picture for the background that was just a black rectangle. Not sure if you tried that if it would color the borders the color of your background pic or if it would override your currents codes you already have or it might do nothing at all.

I think the background image would only go as far as the borders anyway, but as it happens I fixed the issue literally a few minutes ago.

All I wanted to do was remove the horizontal scroll bar from the bottom of the screen, so I added overflow-x: hidden to the CSS body and it's now gone.