User blog comment:BertH/Fluid layout has arrived/@comment-4738647-20131205010026/@comment-452-20131205224304

It is a good idea, and is something you can do using CSS already.

The way the theme designer "splits" the background is that it just applies the background twice (using body.background-dynamic.skin-oasis:after and body.background-dynamic.skin-oasis:before), once aligned to the left, and once aligned to the right, the covers the middle with the fade section.

The trouble is that it's difficult to get it to appear exactly how you want through theme designer, as it pretty much has it's own ideas of where it wants to align the background.

Personally, I gave up on trying to "make it work", and just switched to a tiled background.

edit: Expanding on BertH's comment below, it's would be better to upload a single image containing the "left" and "right" halves, and used CSS to make sure it is positioned correctly.

Perhaps instead, the Theme Designer should be updated to allow the "left" and "right" portions to be manually aligned, instead of doing it automatically.