how can we make an background for the forums?

Sure, you can design a background if you want to—there's not really any particular template, it's about the same as making a background for any other site. If you want it to be a repeating background, then you can make it essentially any size you want (as long as it repeats seamlessly) but if you want it to be a fixed image background that doesn't repeat, you'll probably want to make it large enough that it won't stretch out and look grainy on bigger screens. As for submitting it, you could send it to one of us admins, and we can implement it into a custom theme.

If you're wanting to customize an entire new forum theme entirely, that'd be a bit trickier, since the tools for full theme building are an admin permission. I think it could be possible to make a special "theme designer" role that only has theme editing permissions, if that's what you're wanting to do, I can possibly make that happen

I've been thinking about a custom theme I could make lately and I would love to be able to actually do it. I think a theme editing role would be a good idea, but something else I thought of was just like posting a template or a list of the assets you need for a theme. For example it might be like "to implement a theme we need images for this this and this. the image for the banner should be this big or have this aspect ratio, the image for this should be this and that, so on and so forth."  Then again I don't actually know how this stuff works, so that idea might be impractical, and a role to just have access to the tools seems like an easier option now that I think about it, but that's just something I wanted to throw out there. I think it would be cool for people to be able to submit their own custom themes for funsies.