Alguns temas GTK2/GTK3 para MATE, OpenBox, XFCE ou Gnome3.

Arc

Site oficial: https://github.com/horst3180/arc-theme

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Paper

Site oficial: https://snwh.org/paper/

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Vertex

Site oficial: https://github.com/horst3180/Vertex-theme

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Iris Light

Site oficial: https://github.com/xyl0n/iris-light

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Numix

Site oficial: https://numixproject.org/

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Polar Night

Site oficial: https://github.com/baurigae/polar-night

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Cenozoic

Gnome Look: https://www.gnome-look.org/content/show.php/Cenozoic?content=165734

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Fontes:
https://www.maketecheasier.com/more-gtk-themes-linux/
https://www.maketecheasier.com/gtk-themes-for-linux/