Output brand colours as variables

[richardgaunt]

User story

As a developer
I want output brand colours as variables
So that I can override and reuse these colours

Description

At the moment brand colours are not being outputted as variables.

This means in all the places where we use brand colours we can only return a hex value rather than a variable.

We want to be able to override variables and update colours when we extend CivicTheme and so it would make sense that we add this functionality.

Acceptance criteria

Given I am a developer
When I want to use a brand colour
Then I can use the ct-color-light and ct-color-dark to access brand colours