A beautiful theme that comes with many variations of accent colors and light/dark modes.
Gruvbox Theme
Gruvbox Material theme for GTK, Gnome, Cinnamon, XFCE, Unity and Plank.
Kaupapa Everforest
The idea was born from the need for GTK themes that match the most prominent colour palettes of Neovim code editor and Tiling Window Manager, such as Xmonad, Awesome, DWM, etc, which use these colour schemes to give a uniform and unique look to working environments.
Kaupapa Orchis
Orchis is a flat style gtk theme for Gnome/Gtk desktop.
Kaupapa Pop
A GTK+ theme for Pop!_OS
Kaupapa Skeuos
Dark/Light theme with multiple accent colors.
Kaupapa Raiana
Layan is a flat Material Design theme for GTK 3, GTK 2 and Gnome-Shell which supports GTK 3 and GTK 2 based desktop environments like Gnome, Budgie, etc.
Kaupapa Ataahua
A sweet theme for TROMjaro 
Kaupapa Nordic
Nordic is a Gtk3.20+ theme created using the awesome Nord color pallete.
Windows 10 Kaupapa pouri
GTK theme based on the appearance of Windows 10 using the included dark mode.