Add colour scheme customisation

[joehart2001]

…een/red theme

Pre-review checklist for PR author

PR author must check the checkboxes below when creating the PR.

• I've confirmed the contribution guidelines.

Summary

update table colour from viridis_r to RdYlGn_r

• goes with good/bad thresholds
• more intuiative
• more unique and less like other frameworks

As this is on the topic with thresholds, ive added:

• matching threshold box outer to the colour scheme
• preserve direction of initially set good and bad thresholds: if we set in metric.yml good > bad then try to change bad > good in the app, the good threshold will change to be the bad threshold + our smallest increment (as them being the same doesnt make sense)
• the above also fixes an issue we had when we set good=bad

Linked issue

Resolves #518 #517

Testing

[ElliottKasoar]

This is nice!

The text on yellow for red-green is quite hard to read:

Also, is the selection supposed to persist between visits? It doesn't seem to, but it probably makes sense if it does?

[joehart2001]

This is nice!

The text on yellow for red-green is quite hard to read:

Also, is the selection supposed to persist between visits? It doesn't seem to, but it probably makes sense if it does?

ok yep agree, i hadnt thought of doing that. text wise should be the same way we do for swithcing from white to black for the viidiris table

[ElliottKasoar]

As suggested by @alinelena, let's change the threshold box colours to match this too?