Install adwaita-icon-theme by entering the following commands in the terminal:
sudo apt update sudo apt install adwaita-icon-theme
default icon theme of GNOME (small subset)
This package contains the default icon theme used by the GNOME desktop. The icons are used in many of the official gnome applications like eog, evince, system monitor, and many more. . This package only contains a small subset of the original GNOME icons which are not provided by the Humanity icon theme, to avoid installing many duplicated icons. Please install adwaita-icon-theme-full if you want the full set.