Declare & Use CSS Custom Properties

[SychO9]

Fixes #2600

Changes proposed in this pull request:

• Declare CSS custom properties for Less variables where possible.
• Use CSS custom properties instead of Less variables where possible.
• Declare color variations as CSS custom properties before using relevant mixins.
• Make some hard-coded colors into CSS custom properties.

Reviewers should focus on:
Most of the changes are simple replacements of the common variables. What you should pay attention to is the following:

• root.css declaration of custom properties.
• Backwards compatibility.
• Anything that I might've missed about this.
• Mixins and the way they work (.Button--color(), .light-contents())
  For example the Button--color mixin used to be directly called as such:

.Button {
    .Button--color(@control-color, @control-bg);
}

.Button--primary {
    .Button--color(@body-bg, @primary-color);
}

Now the mixin is called like this:

:root {
    .Button--color-vars(@control-color, @control-bg, 'button');
    .Button--color-vars(@body-bg, @primary-color, 'button-primary');
}

.Button {
    .Button--color-auto('button');
}

.Button--primary {
    .Button--color-auto('button-primary');
}

This is done so that the color variations created by the mixin are declared as CSS custom properties first, as it would make it possible for us to switch to a darkmode with a simple class name change.

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): Some things here should be documented for theme designers and extension devs.

[askvortsov1]

This is huge! Really excited to see us taking this big step towards themability and modern architecture.

[davwheat]

Looks good to me! Just one minor suggestion.

[askvortsov1]

Excellent!