Add information about ui.systemUsesDarkTheme

[outloudvi]

What was wrong/why is this fix needed? (quick summary only): Add link about ui.systemUsesDarkTheme for whoever needs it.

MDN URL of main page changed: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme

Issue number (if there is an associated issue): None

Anything else that could help us review it:

MDN was mentioning ui.systemUsesDarkTheme until #2562. However, there is not only Windows, Android, or macOS platform. For Linux (and potentially other) platform user, they don't have a way to set global theme preference. That's when ui.systemUsesDarkTheme can be useful.

Also worth mentioning that mdn/browser-compat-data#9299 once attempted to take it back to BCD, but it does not belong there. (I also agree so, since this is actually more user-centric rather than developer-centric.)

[github-actions]

Preview URLs

• https://pr4649.content.dev.mdn.mozit.cloud/en-US/docs/Web/CSS/@media/prefers-color-scheme

Flaws

None! 🎉

External URLs

URL: /en-US/docs/Web/CSS/@media/prefers-color-scheme
Title: prefers-color-scheme
on GitHub

• https://blogs.windows.com/windowsexperience/2019/04/01/windows-10-tip-dark-theme-in-file-explorer/ (1 time)
• https://developer.apple.com/design/human-interface-guidelines/macos/visual-design/dark-mode/ (1 time)
• https://drafts.csswg.org/mediaqueries-5/ (1 time)
• https://stuffandnonsense.co.uk/blog/redesigning-your-product-and-website-for-dark-mode (1 time)
• https://support.mozilla.org/en-US/questions/1271928 (1 time)
• https://www.theverge.com/2019/5/7/18530599/google-android-q-features-hands-on-dark-mode-gestures-accessibility-io-2019 (1 time)
• https://www.youtube.com/watch?v=jmepqJ5UbuM (1 time)

[Ryuno-Ki]

This sounds like a great information to add!

[Ryuno-Ki]

Thank you, @outloudvi. Welcome to the MDN contributor community 🎉

[Brazuca228]

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