Qt1 came with two default themes. One of them mimicked Win95 and the other mimicked Motif. KDE1 defaulted to the former in order to look more familiar. To this day, the “Windows 9x” theme still ships with Qt and can be selected on any Plasma 6 install. Starting with KDE2 they started using their own custom themes for everything, tho.

GNOME 1 actually looked very similar, which isn’t surprising because its main goal at that point was to offer a replacement for KDE that didn’t depend on then-proprietary Qt. GNOME 2 and KDE 2 is when they really started building a distinct identity.