I'm going to explain how to change the background of your MyBB forum.
General Background
Go into your AdminCP
Click on Templates & Style
Find your theme, then click on Options and then Edit Style
Find Body in the dropdown
Edit the Background field to your liking. For a solid color, use an HTML color code. For an image, type in url(insert image path here). The image does not have to be on your server in order for it to work.
Click Save
Container Background (where your forums are located, silly.)
Go into your AdminCP
Click on Templates & Style
Find your theme, then click on Options and then Edit Style
Find #container in the dropdown
Edit the Background field to your liking. For a solid color, use an HTML color code. For an image, type in url(insert image path here). The image does not have to be on your server in order for it to work.