As someone who made, designed, and maintain my own website for 15 years, using whatever backgrounds on my own pages I can provide, that was among the first things I questioned on Hubpages. That choice is probably not permitted for very good reasons by HubPages. They store our photos to be viewed on our hubs, which is surely a major storage issue. Think what a massive number of backgrounds they'd have to store for the thousands of hubs! For that, they would probably need to offer a limited small selection of their own backgrounds to choose from (as they do for much fewer instances on profiles, where only one personal choice at a time is available).
Notice there are only two background colors offered (pale blue and pale gray) for individual capsules, and only for capsules placed on the right side position.
The positions of text lines in capsules is limited to the left side with indentation of lines on capsules limited to the one which is accessed on the capsule task bar by the ". Fonts allowed are only one kind, color and size. All the choices we might want mean massive storage for the 'main computer' - Hubpages'.
When I began, indentation was doable. No longer. I saw no others doing the work to get their own choice of indention but perhaps ANY such hubbie-choosing was a burden to the system. Some of my early hubs still have it, though. I can no longer find a way to achieve it.
What we've been offered and supplied here with Hubpages is a pre-processed webpage with choices limited to them, maker of it. It's surely the only viable way it can be offered to literally tens of thousands of members! I shudder to think of the technical expertise and work it must take to keep it running smooth as it is!
My brief experience with using other pre-processed webpage setups was the same. They offer their choices & selections and a few freedoms of their choice and that's it. It's why I 'do it myself, ma' on my own website.
So - I was accustomed to doing whatever I was capable of doing or figuring out with my minimal HTML on my own site. I loved the freedom of it. It meant trading that freedom for being on Hubpages and using what they offer. They do offer much that no individual can make happen.
I've learned to accept and appreciate what HP provides which represents a lot - an audience, lots of help and incentives, and what they choose to provide in the way of esthetics and a few ways for us to use them to suit ourselves.
I'd say that it's very doubtful that they'll start permitting optional backgrounds, even to color selection.