Theme only works on private window

[JoaoFelipe3]

I've placed all the files in the correct folder, but only userContent.css seems to work; on a private window the theme works properly.

[dukleinerfratz]

Had the same issue. I'm running Linux with Wayland (hyprland) so if you're on a different platform this may not for you.
How I fixed it:

1. Download the CSS files and place them in the chrome directory
2. Enable dark mode in Zen
3. Install 'xdg-desktop-portal-hyprland' and 'xdg-desktop-portal-gtk'
4. Log out and back in again