Install blackbird-gtk-theme by entering the following commands in the terminal:
sudo apt update sudo apt install blackbird-gtk-theme
dark GTK+ theme from the Shimmer Project
Blackbird is a dark variant of the popular Bluebird and Greybird themes from the Shimmer Project. The Shimmer Project created the default themes for Xubuntu since 2009. . Although originally created for Xfce, Blackbird supports other GTK+-based environments including GNOME Shell, MATE, Cinnamon and Xfce.