Wine actually has (mostly) functional theming support-- point Wine to a .msstyles theme DLL and it'll draw themed controls (rather than the Windows Classic style default).
They just don't ship a theme with Wine; understandable for licensing reasons (they can't distribute Microsoft's designs), but it means that Wine looks like Windows 2000 out of the box.
I find it a bit amusing that were they to "update" their look to that of the flat Windows 8+ UI, it would probably entail the removal of a lot of code.
...and yet even on a 25MHz 386, no one complained about the slow drawing speed of the "fancy" 3D buttons in Win3.x.
Well, modern flat design generally also includes a much more animated interface, which would probably entail the addition of much more code. And I don't think the drawing speed of buttons was ever a very strong reason for the rise of flat design - though I completely understand your distaste for it.
They just don't ship a theme with Wine; understandable for licensing reasons (they can't distribute Microsoft's designs), but it means that Wine looks like Windows 2000 out of the box.