In the most recent nightly build, the browser is set, by default, to look to the OS theme first to determine whether to use a Light or Dark theme. While this may be appropriate on OSs that allow for scheduled OS theme changes, many OSs, including Linux Mint and Windows 10, do not offer this natively. At best, external applications allow this change, and at worst the only option is for users to "hack" the OS to enforce scheduled theme changes.
In these scenarios, the default behavior of Firefox is unacceptable.
A popular extension exists called AutomaticDark, which allows for scheduled theme changes within Firefox (besides the default Light/Dark), and these theme changes can be triggered either by the OS theme changing or by independent triggers (sunrise/sunset, custom time, etc). What this extension currently does not take into account is whether the browser recognizes Light or Dark theme as applicable. Without going into about:config to change the settings, the browser will still act in "Dark" mode if the OS is in "Dark Mode" but AutomaticDark has it set to "light" mode.
Two solutions exist to this paradox.
1. Provide a proper GUI to configure this setting, perhaps with expanded options to natively change the theme as AutomaticDark would otherwise be necessary to enforce.
2. Expand Add-On permissions to access and change the "Light Mode/Dark Mode" configuration, with AutomaticDark in mind.
Either solution will allow for stronger customization of the browser independent of how the OS may be configured.
An example of how this is already applied can be seen in Vivaldi, where Theme Changing is built into the browser natively.
Well, it looks like the lastest Nightly browser has an official GUI for controlling this instead of About:Config, so I can only assume based on how little attention this Idea seemed to get that you're two steps ahead of me.
Regardless, thank you.
For the eyes, it is useful that in the dark, automatically in the browser, the theme changes from light to dark, it will be useful!
Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.