Random dark colour generator

Generates dark colours — low lightness with usable saturation — for dark-mode backgrounds and anything that needs light text on it.

The one piece of advice worth taking about dark interfaces is not to use pure black. A true `#000000` background against white text produces a contrast ratio around 21:1, which is high enough to cause halation — the visual smearing where light text appears to bleed into the background, particularly for readers with astigmatism. Material Design recommends a very dark grey around `#121212` for exactly this reason. Dark colours with a little saturation also read as designed rather than as an absence of colour, which is why good dark themes are rarely neutral grey.