If you feel designer-y today, I can give you the current forum style, and a link to the Xenforo free demo. In that way you can mess around with it yourself

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Is that an offer to let us try making or altering a theme? I used to do a lot of that on the InvisionFree forums (I'm very proud of the Classic Doctor Who theme I created for the Tom Baker forum I admin'd).
Are there any free themes we could try?
I rather liked some of the themes available for various forums 15 years ago. The first forum where I was co-admin was hosted by Forumer, and we had over a dozen themes. I remember one day when I thought I'd just try all of them for awhile to see which ones I liked best. Apple Green was the winner, as it was both cheerful and relaxing. It was funny how the guys didn't want me to delete the pink one... it was like a princess pink, but they wanted to keep it.
I am not sure if we can easily do something about the image elements, and I actually like them

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Okay, not cool (a problem I just found, not your opinion even though I disagree with it

). I want to do separate replies to each post since they are not the same post. There's no room between and I can't insert a line. This will make multiquoting an absolute nightmare. Apparently I have to go into BBCode for this (annoying on ProBoards; didn't think I'd have to do it here).
Anyway, images. There should be a place in the theme section of the ACP where you can swap out the default icons and use other ones. There are whole libraries of sets of icons available online (not sure if anyone's made any especially for XenForo). Wouldn't it be cool to have a Civ-centric set? (if one doesn't exist now, I'm sure there are artists here who could make one).
As for the issue of non-collapsing sidebar stuff... I kept them collapsed even though my screen is magnified to the point where all that stuff is actually at the bottom so I usually forget it's even there.