[em_te]

I think windows were flat on Windows 3.1 too and only adopted the 3D raised borders starting with Windows 95.

[p_l]

Windows 3.0 brought "3d" styling to windows, actually - where available at least. Windows 2.0 had some simple shading or rather "complex borders" if you had colour display, but nowhere close to 3D shading of 3.0