How to reset Ubuntu 18.04 to default theme? [duplicate]
It may be easiest for you at this point to reset your theme and icons to the Ubuntu factory defaults with two command in the terminal:
gsettings reset org.gnome.desktop.interface gtk-theme
gsettings reset org.gnome.desktop.interface icon-theme
Open a terminal, copy and paste these two commands and your theme and icons will be reset to those of a freshly installed Ubuntu system. To change to other installed themes later on, you can use Tweak Tool.