Hi,
If you’re not using GNOME
If you want to change the theme of Libadwaita applications on another Gtk desktop like Xfce or Cinnamon, you can use the following commands:
Light Adwaita theme:
gsettings set org.gnome.desktop.interface color-scheme default
Dark Adwaita theme:
gsettings set org.gnome.desktop.interface color-scheme prefer-dark
…and welcome to the forum !