I believe Win9x all had basically the same UI appearance, and 2k just changed the default colours slightly (and they were always fully customisable, so no difficulty in changing them either.)
Windows 1.x/2.x were noted for being rather primitive, and similar to the "flat design" trend hated by many when Win8 came out. 3.x is when the UI elements became 3D, and then they retained nearly the same appearance until XP introduced the bitmap-based theme.
Windows 1.x/2.x were noted for being rather primitive, and similar to the "flat design" trend hated by many when Win8 came out. 3.x is when the UI elements became 3D, and then they retained nearly the same appearance until XP introduced the bitmap-based theme.