accounts/styles and themes gives you the options in the drop down box to either create your own, use the default chess.com theme, or browse others themes.
To create your own, click where it says "create a new theme" it will open a new page where you can change page colours, add a new picture in the top bar .. all kinds of things.
So... I'm thinking about creating a theme for myself, but I don't really know how XD
I couldn't find any help guide either ._.;;