Install greybird-gtk-theme by entering the following commands in the terminal:
sudo apt update sudo apt install greybird-gtk-theme
Description:
grey GTK+ theme from the Shimmer Project
Greybird has been the default theme for Xubuntu since 2011 when it replaced Bluebird. Greybird can be described as more conservative yet more modern than Bluebird. The Shimmer Project are the designers and developers of both themes. . Although originally created for Xfce, Greybird also supports other GTK+-based environments including Cinnamon, GNOME Shell, MATE, and Unity.
Homepage: https://github.com/shimmerproject/Greybird
Version: 3.22.8-1
Section: universe/x11