I tried to make a GTK theme like that once. It sucked, not in the least because Win 3.x UI components are terrible at consistency: here you have 3d buttons, but other elements are 2d, here's this thing 1 pixel wide, and another 2 pixels wide for no reason at all, and here are whole classes of widgets that plain don't exist and every program has to roll its own (toolbars, for example; also look at Borland IDEs of the time, and MS Office too).

Windows 95 was quite revolutionary because it brought in a very consistent look and feel to the UI.