I like the new display theme, but I really wish you could turn off that stupid floating toolbar thing at the top. I turn off all of the normal toolbars because I'd rather have the screen space, I sure as heck don't want that stupid floating thing!
It also annoys me that you can't put a toolbar on the same level as the File/Edit/... menu anymore. I like to have the Filter toolbar in PCB Editor, and in AD17 you could put this (or any toolbar) in the top menu row where there's a bunch of dead space anyway. Now that they've added that extra group of icons on the top right you can't do that, so adding any toolbars means adding another menu row which means more dead space.