Opening a private window causes Firefox's auto system theme to switch to dark mode

[nekohayo]

With Firefox 104, on Linux, if the Firefox theme is set to the default ("System theme — auto"), even while my system's theme is in light mode (regular "Adwaita" theme), when I spawn a new private Firefox window with your extension enabled, it turns the non-private main window dark, as if the system theme had been switched. Basically, the opposite of issue #6!

[hmijail]

Similar case in macOS 14, as of Firefox 122.

My setup is simpler, since my theme is just set to light; but opening a private window changes the theme to dark in all windows.
The theme stays dark when the private window is closed.
The theme only turns light again when opening a new normal window.