[2.x] feat: add dark mode logo support

[rafaucau]

Changes proposed in this pull request:

Not every logo works well in both light and dark mode. For example:

Light mode	Dark mode

This PR adds an option to set an alternative logo for dark mode.
If no dark mode logo is set, the regular logo is used in dark mode.

Screenshot

Light mode	Dark mode

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.
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[imorland]

Nice, will look over this!

[imorland]

Nice feature, clean implementation. A few notes:

Change request:

The dark logo <img> has no base display: none rule, so browsers without :has() support (notably Firefox < 121) will render both logos simultaneously. Adding:

.Header-logo--dark-mode {
  display: none;
}

as a base rule would give correct graceful degradation — those browsers would simply always show the regular logo.

Notes (non-blocking):

• The dark logo is not used in HTML mail templates — email dark mode support is too inconsistent for this to be practical, just worth documenting as a known limitation.
• [data-theme^='dark'] correctly covers dark-hc (dark high contrast), so high contrast users get the dark logo. No separate variant is possible, but that's a reasonable limitation for now.
• No tests, consistent with the existing light logo situation — not a blocker.

[rafaucau]

@imorland .Header-logo--dark-mode is already hidden by default.

https://github.com/rafaucau/flarum-framework/blob/9498eb2101e01b1323ec77de4c2370eae00ac1b5/framework/core/less/common/App.less#L187-L189

So in browsers without :has() support, this will gracefully fall back to the current behavior and continue showing the regular logo.

[imorland]

Apologies for the confusion — you're right, the base rule is already there. Change request was incorrect. LGTM!

[imorland]

Apologies for the confusion — you're right, the base display:none rule is already there. Change request was incorrect. LGTM!