For our colour system, we standardise on perceived brightness in order to have readable contrast no matter which colour you pick.

We have an exception for step 9 of the scale (which is used for the main branding elements, like buttons or primary text) to use the chosen colour exactly unless it is wildly off the mark in terms of readability. In some cases, specifically shades of yellow, the contrast is often too low and is replaced with a more contrasting colour. This results in some pretty dark-looking UI elements for buttons and steppers.

This PR introduces an extra colour step for backgrounds, bg-*-original, which will always be the originally chosen colour. The existing bg-*-solid remains available for use cases where the colour should be guaranteed to be readable.