What you're looking for is a skybox. I think there is already a skybox by default, but I might be wrong. Try holding down Ctrl and scrolling your mouse wheel down.
If it doesn't appear, make sure your texture set has one set. First you need to know which terrain set you're using. Maybe you're using Char or some urban tileset or whatever.
Open the data editor, open the Texture Terrain Set object type and go to your texture terrain set(with the same name as your texture set). You can set the skybox in the Background Model (Fixed) field.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory