Here's the link to where the background is hosted. Just do object>background>custom as normal, and then use
https://emergentlandscapes.files.wordpress.com/2012/05/the-witcher-pano_-8.jpg

Very cool background

Okay, the tutorial from 2015 no longer applies so I figure I'd give an update, since it took me some time to figure it out for myself. From inside another game, click on the 'Games' button at the top of the screen, then click the big blue 'Workshop' button on the left side of the popup. You should see 'Dark Forest Background' among the workshop items listed. When you hover over it, a button with 3 vertical dots should show up named 'Options.' Left-click the button and a drop-down menu should pop up. Choose 'Expand' which should lead you to another screen that simply has a button labeled 'Custom Background' with a generic icon. Simply click on the icon and that should load the background into your game.

Like BunBun said below, this is not intuitive at all; I spent a while mucking around in the 'Objects' -> 'Backgrounds' menu before I realized I was looking in the wrong place. >_>

@Alpine Thanks for the 2 year old tutorial below. It lead me to the right place in the current version.

Totally NOT intuitive, jesus christ who ever thought hiding that setting there was a good idea needs to be denied beer for the rest of their lives.

@BrokenContender - I posted a comment further down that explains how to use backgrounds. Hope that helps!

so ive made a custom deck and would like to have this be my background but i really don't understand how to do it. Would anyone please walk me through it? Nothing seems to be working

NICE

Really nice

This looks great. The only thing missing would be a camp fire.

GREAT

You should be able to use it in combination with other mods by first loading the other mod(s) and then bringing in just the background. To do this, open up the Workshop menu and select the triangle next to the background. Then select 'Custom Background - Regal'. This will import the background as an entity seperate from any other mods that are already loaded. Here is a link to a picture explaining: http://i.imgur.com/vSFafNq.jpg

Thumbs up, looks cool.