Nordson is a Gtk4 theme based on the awesome Nordic Gnome theme.
Extract the zip file to the themes directory i.e. /usr/share/themes/ or ~/.themes/ (create it if necessary).
To set the theme on Gnome, run the following commands in Terminal:
gsettings set org.gnome.desktop.interface gtk-theme "Nordson"
gsettings set org.gnome.desktop.wm.preferences theme "Nordson"
or Change via distribution specific tweak tool.
If you're a firefox user you should give a try to the awesome Nordic theme for firefox.