ColorPalette: Contrast checking doesn't work with CSS variables

[mirka]

The ColorPalette component uses the contrast() function of the colord library to decide whether to render the text label of the color in black or white.

This does not work when the color value is a CSS variable. We'd probably need to do something like getComputedStyle( bgElement ).getPropertyValue( '--my-color' ) to get the actual color value to pass to colord.

Screenshot