@Gavin - This worked for me by pasting it into custom CSS - just replace the mymoosocialsite.com with your site, and make sure the https or http is set how your site is too.

 

body[id^='page_guest'] {
padding-top: 125px;
background-color: red;
background: url('https://mymoosocialsite.com/theme/default/img/redlightback.jpg');
}

@Gavin - This worked for me by pasting it into custom CSS - just replace the mymoosocialsite.com with your site, and make sure the https or http is set how your site is too. And upload the image to where it needs to be :) 

 

body[id^='page_guest'] {
padding-top: 125px;
background-color: red;
background: url('https://mymoosocialsite.com/theme/default/img/redlightback.jpg');
}