Webpage background color

How do I change the background color ?
I couldnt find any settings for this


See https://forum.xojo.com/25923-set-background-color-of-page-in-code

But you can also create a webstyle with a background color and apply that to the WebPage.

I dont understand this webstyles, bear with me Im new to webapps.
Could you post an exaple ? Id like to Turn the background to dark grey wich is easier for the eyes