Steam Customizer like many, is just an add on for steam.
There are many more popular ones including Enhanced Steam, Steam Inventory Helper, Fair Steam, etc.

Steam Customizer is a website that allows you to download backgrounds for your Steam.
But instead of your profile it decorates your entire Steam!

Steam Customizer allows you to change the colors for alot of things.
It also allows you to add a custom picture.

In Steam Customizer's menu there are alot of user created backgrounds.
When you first enter Steam Customizer the most popular backgrounds will be shown to you.

In Steam Customizer, downloading a background isn't as simple as clicking download on a background.
You would first need to download their App.

You can easily find this here

Proceed to click "Download Tool"

And it will take you here

Now finally download it.
Once you do that it will make a desktop shortcut and it will allow you to choose which background you could use.

But wait you have no background?
Simply quit the app, go back to the browser and pick your background!

Here is the one for TIMBERFOX

If however this does not go succesfuly, please comment down below and I will guide you personally on how to fix your problem.

This has been made to promote the background for TIMBERFOX. And I used Wallpaper Engine since its the closest thing to a wallpaper

If you want to learn how to make one aswell, add me as a friend and I will teach you!