Generates 'gradient-dark' automatically & checks colour contrast

1. Brand colors generator:

• Preview and copy the hex code of 'gradient-dark' simply by providing a 'brand-primary-color'.
• Devs can assign a 'brand-primary-color' either from a selection of default colors or manually inputting the hex color.

2. Color contrast checker:

• Checks the auto-generated gradient-dark against our non-branded color variables (ff-white, ff-light-grey, ff-alice-blue, ff-grey, ff-grey, ff-dark-grey, ff-black) under 4 accessibility standard situations (levels "AA" and "AAA", font size "small" and "large").
• If the color contrast is not ideal, devs can use Sketch color picker to manually pick a lighter/darker shade of the 'gradient-dark' color and check again until it meets the WCAG standards.
  When devs manually pick a gradient-dark color, the 'brand-primary-color' will be updated in Brand colors generator section.