Colour calc

			Text colour
			-----------
			Against solid backgrounds this works fairly well:

				hsl( from var(--row-colour)    h s calc(sign(l - 51) * -100) )

			But doesn't work for some lighter and mid-tone colours like lime, cyan, and yellow.
			Lab colour space works better for this:

				lab( from var(--row-colour)    calc(sign(l - 70) * -100) 0 0 / 1 )

			General
			-------
			Lightness Inversion:
			hsl		h s calc(100 - l)

			Darken:
			hsl		h s calc(l / 2)

			Lighten:
			hsl		h s calc(l + (100 - l) / 2)