Install bluebird-gtk-theme by entering the following commands in the terminal:
sudo apt update sudo apt install bluebird-gtk-theme
blue GTK+ theme from the Shimmer Project
Bluebird was the default theme for Xubuntu 10.10 but since 2011, Greybird has been Xubuntu's default theme. As its name indicates, Bluebird is a blue theme. The Shimmer Project are the designers and developers of both Bluebird and Greybird. . Bluebird works best in Xfce since it currently has incomplete support for newer versions of GTK+ 3.