[BUG] Thorium misidentifies the light/dark mode of some system-wide themes.

[yum13241]

System Details

• OS: EndeavourOS
• Thorium Version is 128.0.6613.189.

Problem
Thorium doesn't hint to websites that the user prefers dark mode, and if I use "Classic" (instead of "GTK" or "Qt"), the browser becomes light until I choose "Dark" instead of "System".

Additional Notes

1. Right-clicking Thorium and checking "Open in Dark Mode" doesn't work.
2. This command line: /usr/bin/thorium-browser --force-dark-mode --enable-features=WebUIDarkMode %U (the %U doesn't matter here) does not work. Adding --temp-profile to the beginning of the cmd-line args doesn't work either.
3. No amount of chrome://flags works. ("Auto Dark Mode for Web Contents" is a primitive Dark Reader and doesn't solve the problem).
4. I use this theme (simplewaita with GTK+Kvantum for Qt apps)

Reproduction Steps

1. Open Thorium
2. Go to https://thorium.rocks
3. Get blinded (if you aren't, clear the website's cookies)

If it didn't work, try these steps:

1. Use the Simplewaita system theme. Thorium correctly detects Adwaita-Dark as a dark theme, for example.
2. Use EndeavourOS or any Arch derivative (except Manjaro).
3. Use the gtk3-classic package from the Arch User Repository. (probably has no effect)
4. Don't use the "Classic" theme, try GTK or Qt instead.

System Information

You may also want to see khongten001/Simplewaita#2.