GTK+3 and GTK+2 themes base on : Arc-Theme .
To use this theme requires GTK+ 3.22 installed in a computer system.
This themes have been test on Linux Debian 10 (Buster), Ubuntu 18.04.3 and Linux Mint 19.2
Available in Light & Dark variants.
Download themes on Files Tabs or click Download Button on the right sidebar and then, extract files on directory /usr/share/themes as root.
For Gnome Desktop User visit this page Seventeen Gnome-Shell Themes to download themes.