How do I add a background on the signage?

You have two options, one is to put a view at the back of all the screen divisions that you have.

1.) Click on Screens
2.) Go to the screen division that you are using
3.) Add another “view” on the screen division
4.) On the Screen Templates Panel drag the screen division that you added to the top so its the first item in the screen divisions.

OR

Create a background mask and put it anywhere since the background mask will have holes for the items in your campaign.