Fixes #0000

Changes proposed in this pull request:
Changes the LESS/CSS code to be able to switch color schemes using Vanilla CSS (browser level) rather than having to recompile all of the LESS code based on configuration values.

This also means the ability to easily have per-user color scheme preferences, so this PR also adds the ability to select a color scheme per user while checking the user system's color scheme preference as default.

This also removes the @config-dark-mode and @config-colored-header less variables as they are no longer used and should no longer be used in extensions in 2.0.

Instead one can do:

[data-theme=dark] {
  .my-element { ... }
}

[data-colored-header=true] {
  .my-element { ... }
}

Additionally, this will allow easily adding high contrast color schemes and the like for a11y (accessibility).

Screenshot

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)