There's more cleanup I'd like to see, but this ticket is specifically for colour scheme info.

Suggestions:

• Remove all the less functions that are generating colours with things like "lighten" etc, replace with plain hex colours
• Remove all the instances where we have alpha values on colours that aren't actually meant to be over different backgrounds, replace with plain hex colours
• Audit the list of colours declared in `theme.less` and `variables.less`
  • Remove everything we're not using
  • Extract into a "colour scheme only" include - with comments about where they should be used
  • Enforce a (short) list of specific greys
  • Make random threats of bodily harm on anybody wrecking up (or ignoring) said new colour bible