Ah, I remember RMRK1/2/3...
I don't know why I was not fond of RMRK5. I think it feels cluttered to me with a lot of small text and a lot of information. Putting the user panel at the top of the page and having content a single column makes a huge difference. The forum names being larger also helps draw attention to them.
The one thing I'm unsure about is the format of post title times: the relative-style is a little confusing to me, though it does lend itself to being tidy. I think posts yesterday should have a "yesterday" flag, I'm seeing posts here (by my time zone) posted at 6PM, and I have to pause to realise, yes, it means yesterday (since it's 11am here for this post).
So yeah, really great job this theme has. I don't think I see anything else to comment on at this time, design wise. Function-wise I'd do away with reputation too but I don't think I can think of anything constructive to say on the subject.