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Flat? Where? I think it's just like Server 2003 or XP Classic theme. Is there a flat UI theme?

Look at the gallery, most screenshots use a flat theme: https://reactos.org/gallery/

Oh, you mean these themes, Mizu and Lunar: https://www.omgubuntu.co.uk/2019/09/react-0-4-12-released-do... They look like flat UI but not exactly a Windows 8 or 10 way. Well, at least they are optional.

It is just a theme, it is switchable ^)