Improve colour contrast A11y by defining colour pairings for text

Currently theme colours are given as primary, secondary and tertiary colours.

This means that the contrast between text colour and background colour are not necessarily considered when creating the theme.

Proposal:

• Instead of thinking of text in terms of the colour of the text, we should move to considering text as a colour pairing of colour of text and its background (what colour ink to use on what colour of paper, rather than colour of ink alone).
• Theme colours should be specified in pairs, and each pairing could be checked for contrast against the WCAG standards when chosen.

Practical considerations:

• should text colour pairs be defined explicitly as settings?
• shall we have Janeway report the contrast ratio for that pairing whenever it is updated, and warn if it is below standard?
• should all theme colours be defined as pairs, such that the appropriate colour for any text against that background is always known, or should text colour pairings be a subsection of theme colours?