Is this related to the detection of dark theme?

Check what the pannel shows. If it says a dark theme is detected, but the dark reader is still enabled, then it's a known issue #11990, which seems almost always happening on newly opened tabs.

Please see:
https://github.com/darkreader/darkreader/blob/main/CONTRIBUTING.md#disabling-dark-reader-on-your-site

Fixed for me.

This is fixed

I think this is not the intention of the megathread, you may open a new discussion for it.

BTW, your first thought probably can be implemented like getComputedStyle(document.documentElement)['color-scheme'].includes('dark') as a part of the dark theme detector, so if the page can be rendered in dark color scheme, DarkReader won't activate. The pro is the standard feature allowing the detector to work on more websites than the meta tag. However, the current detector based on background color seems fine, so I wonder why you need to tell DarkReader manually rather than letting it detect the background color automatically.

The webpage in question has no colors at all, instead relying on the browser defaults that are keyed by the color scheme ... so there's nothing to detect if Dark Reader is ignoring the stated color scheme. Employing getComputedStyle(document.body) may solve that, though it'll be fooled by sites that have a dark body as a border around a light <div> that contains all of the content.

Seems always dark. I will add this URL to Dark Sites. 😉

Added entry in #14153

1. Can you make some screen capture video and share it here?
2. What browser you use because I can not reproduce your described problem on Firefox.

Re 1. Screen capture video: https://github.com/user-attachments/assets/f0f10f94-0c52-4fc3-b767-5a33652f7a93

Re 2. I use Chrome Version 139.0.7258.139 on Windows 11.

Still can not reproduce but I am adding this site as NO DARK in detections configuration.

#14608

Thanks for info. Added detector to detector-hints.config in #14694

Added detections in #14818

I will leave this untouched as I do not have any valid account to work. Maybe someone will have some already and could check what can be done with it.

For {account, mail, calendar, drive, wallet}.proton.me their identifier is [data-theme-mode="dark"] on the html element.
The rest of services are a bit wonky with theming (they use css classes on body, or a style element with an id, etc).

I can add detection when Dark Theme is manually set in setting. System preference can be dark or light so I can not include this in detections.

Added detection in #14819