Currently the header text colour changes only the header text. Let's set it as a secondary colour used in a few different places so that we can tie the colour scheme together and add a bit more variety.
[x] Rename the customizer option to 'highlight colour' (or something similar?)
[x] Create a custom property for the colour
[x] Update the custom property in the customizer-preview.js
Currently the header text colour changes only the header text. Let's set it as a secondary colour used in a few different places so that we can tie the colour scheme together and add a bit more variety.