Use our own colour palette across the UI

[LeoMcA]

Upstream discussions:

• https://meta.discourse.org/t/cleaning-up-our-color-palette/61263/18?u=leomca
• https://meta.discourse.org/t/override-global-sass-variables-from-theme/68146

Currently Discourse takes the colours we define and lightens/darkens them in various areas of the UI. We want more control than that, and once we have it:

• Try applying blue 3 everywhere, make sure it doesn't make everything too electric
• If it does, try to use blue 2 everywhere, not allowing the inbuilt Discourse styles to lighten it in various places.
  • Dim things on hover
  • Try to use family over opacities

Or try applying blacks/greys everywhere? From https://github.com/mozilla/discourse/issues/94#issuecomment-326089672:

Additionally, UI across the site could become black, white or shades of gray. e.g. the user notification counter could be a black circle with a white number. You're nearly doing this across the site as it is, why not push it further and make a rule? This fully allows category colors to exist without competing or being disruptive.

┆Issue is synchronized with this Jira Task

[LeoMcA]

We've done a certain degree of this with the DinoPark update to the theme, but https://github.com/discourse/discourse/commit/d348368ab6790be8f69b8d3bc4352c96aaab73c3 gives us way more control (particularly in the dark theme)

[data-sync-user]

➤ Leo McArdle commented:

More documentation about this here: https://meta.discourse.org/t/override-values-for-auto-generated-color-variables/134374