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)

[SychO9]

Thanks for the PR!

The reason auto is used is so that the JavaScript code can determine whether to use the user-preference or user browser color scheme.

If data-theme can no longer be auto it would undo that logic (see the setColorScheme method).

[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:

output2.mp4

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After:

output.mp4

[SychO9]

Oh sorry, you're absolutely right!