If you leave boxes that are put in using HTML, not CSS, in Easy Customization blank, then they'll revert to the main board page's.
The ones that use HTML are the borders & table background ones. What you can do is either enter "none" for the colour, or " ", or just choose a colour.
Easy Customization is designed for you to customize all of your board/forum, not just one part of it, like the background.
If you just want a completely blank page, and a background, then you can disable EC, and use the CSS:
If you want it to be a background image, use: