Background
In T328850 we made dark mode tri-state in Vector 2022 and Minerva skins.
We should copy the approach here in the third party dark mode extension.

Feature summary:
Browsers report the user’s preferred color scheme (dark/light) via media query.

• The behavior should be changed to allow for deriving the color scheme from that media query instead of choosing an option manually.
• The UI should be changed to allow switching between three options: “automatic”, “light”, and “dark”. Options for the UX are e.g. the “Theme” dropdown on https://developer.mozilla.org/en-US/, three radio buttons, or a button cycling through the three options.
• The default should be “automatic” to follow the system wide setting (available in Windows 11, macOS, and most Linux desktop environments).

Use case(s):

• I have my system set up to sync color scheme with daylight. I want all websites with dark/light toggles to honor this automatic switch at sundown/sunrise.
• People with sensitive eyes need dark mode. Following the system scheme by default prevents a flash of light hurting them.

Benefits:

• Less clicking around and searching for UX for users who have set a preference globally.
• Less pain for people relying on dark mode for health reasons.
• Only way to support daylight dependent dark/light mode.