Option to do colour scale for whole theme, rather than just individual colours

## Description

Used to be able to do this in previous version but I can't seem to find a way to get it to work now.

## Why do you need this feature?

I found it really handy to see all the color scales together on one page to suss out which colour space and scales were best for all colours at once.

## Additional context

Also did really like being able to change the colour space for individual colours and see how each compare within the theme - I used this to suss out the colour scales I liked best.

No files, tests, or entry points are identified in the issue. First compare the current theme and individual-colour workflows with the previous version, then define the expected whole-theme colour-scale view and confirm that all colour-space comparisons work together.

Written by the indexing model from the issue text.