I want to change the web page background color.

I want to change the web page background color.
The current theme only has 1 default color which is white, even though I tried to change the color in Customize but it doesn’t work?

Please help me change the background color of the interface or png so that I can upload my own background image to replace the default White color. Thank you

This is my website: https://nghiensoikeo.com/

and here is the reference interface: https://baocsn.com/

The page I need help with: [log in to see the link]

 

This site will teach you how to build a WordPress website for beginners. We will cover everything from installing WordPress to adding pages, posts, and images to your site. You will learn how to customize your site with themes and plugins, as well as how to market your site online.

Buy WordPress Transfer