Add colour to theme colour palette

[jonnywatersbb]

What problem does this address?

Currently no way of adding a new colour to the palette for use within the theme

What is your proposed solution?

Replicate the flow for adding a custom colour within the Style Book, but add to the theme colour palette:

[g-elwell]

I've started a discussion on how we could support this alongside other settings in #94 - closing this so we can keep the conversation in one place. I've proposed a different approach for the UI but would appreciate your input on that issue, especially around things like the colour palette.