fix: respect user theme preference

[datlechin]

This PR fixes the issue where data-theme defaulted to auto, causing a flash of light mode before switching to dark. It ensures the theme respects the user's preference for a smoother experience.

Fixes #0000

Changes proposed in this pull request:

Reviewers should focus on:

Screenshot

QA

Necessity

• [ ] Has the problem that is being solved here been clearly explained?
• [ ] If applicable, have various options for solving this problem been considered?
• [ ] For core PRs, does this need to be in core, or could it be in an extension?
• [ ] Are we willing to maintain this for years / potentially forever?

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).
• [ ] Core developer confirmed locally this works as intended.
• [ ] Tests have been added, or are not appropriate here.

Required changes:

• [ ] Related documentation PR: (Remove if irrelevant)
• [ ] Related core extension PRs: (Remove if irrelevant)

[datlechin]

@SychO9 The logic behind auto is still preserved. If color_scheme is set to auto, it falls back to the user's browser preference via RequestUtil::getActor()->getPreference('colorScheme'). This ensures the same functionality while preventing the flash of light mode.

[datlechin]

Before:

https://github.com/user-attachments/assets/e197bf84-41de-419d-b0a5-43f089089d94

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After:

https://github.com/user-attachments/assets/3c812b68-d6ae-41d6-b41c-c37f1aa050b9