Run the following commands in a terminal
# update repositories
sudo apt update
# add the raspberry pi os gpg key
wget -O - http://archive.raspberrypi.org/debian/raspberrypi.gpg.key | sudo apt-key add -
# add the raspberry pi os apt repository
echo deb http://archive.raspberrypi.org/debian/ $(lsb_release -cs) main | sudo tee /etc/apt/sources.list.d/raspios.list
# update repositories
sudo apt update
# install pixel
sudo apt install lxpanel pcmanfm openbox xserver-xorg x11-xserver-utils policykit-1 lightdm raspberrypi-sys-mods zenity libglib2.0-bin desktop-file-utils lxsession adduser mutter xdg-user-dirs raspi-config xserver-xorg-video-fbturbo fonts-piboto pipanel lxinput pi-greeter rpd-plym-splash rpd-wallpaper pishutdown scrot gtk2-engines-pixbuf gtk2-engines-clearlookspix gnome-icon-theme pixflat-icons lxplug-volumepulse lxplug-network lxplug-bluetooth lxplug-ejecter lxplug-ptbatt rc-gui gtk2-engines-pixflat lxplug-cputemp lxplug-magnifier rp-bookshelf agnostics gui-pkinst cups system-config-printer pi-printer-support lxplug-updater raspberrypi-ui-mods -y