A beautiful theme that comes with many variations of accent colors and light/dark modes.
Gruvbox Thema
Gruvbox Material theme for GTK, Gnome, Cinnamon, XFCE, Unity and Plank.
Everforest Thema
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.
Orchis Thema
Orchis is a flat style gtk theme for Gnome/Gtk desktop.
Pop Thema
A GTK+ theme for Pop!_OS
Skeuos Thema
Dark/Light theme with multiple accent colors.
Layan Thema
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.
Séiss Thema
A sweet theme for TROMjaro 
Nordic Thema
Nordic is a Gtk3.20+ theme created using the awesome Nord color pallete.
Windows 10 Däischter Thema
GTK theme based on the appearance of Windows 10 using the included dark mode.