[Linux] fix theme updates on GNOME 42

[jpnurmi]

Prefer org.gnome.desktop.interface.color-scheme for determining the
UI color scheme. The new setting is available in GNOME 42 and later.

https://blogs.gnome.org/alexm/2021/10/04/dark-style-preference/

Fixes: flutter/flutter#101438

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt.
• I updated/added relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[jpnurmi]

theme.mp4

[robert-ancell]

Other than the reference issue LGTM

[jpnurmi]

Depends on #32618

[flutter-dashboard]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

[jpnurmi]

(rebase only)

[jpnurmi]

Superseded by a more generic XDG desktop portal based implementation:

• [Linux] read settings from XDG desktop portal if available #33100