[1.x] Custom Colorising with CSS Custom Properties

[SychO9]

Fixes #2816
Progresses #2600

Changes proposed in this pull request:
This PR introduces usage of CSS variables instead if custom colors passed as inline styles, especially as a background property. The PR is not meant to convert all of the codebase to use CSS custom properties, that will take place in another PR(s), where the root CSS variables will also be properly organised.

The changes to the sideNav links and button colors are done for the tags extension's custom colorising.

This should be 100% backwards compatible and should not break any custom styling in existing communities.

Reviewers should focus on:
General code quality and anything I missed.

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

Required changes:

• Related core extension PRs:
  • [1.x] Custom Colorising with CSS Custom Properties tags#139
  • [1.x] Custom Colorising with CSS Custom Properties sticky#29
  • [1.x] Custom Colorising with CSS Custom Properties lock#29
  • [1.x] Custom Colorising with CSS Custom Properties subscriptions#42

[davwheat]

Thank you. Thank you! THANK YOUUUU! 🥰