Add dark mode as accessibility for vision-impaired users.
Some user's visual processing is more accommodated by inverting all colors. Because white-background-based color schemes have usually been thought out and debugged (as not causing difficult-to-see lacks of contrast or lacks of hue), accessibility in this topic usually XORs the RGB color (think the hexadecimal form) with 0xFFFFFF, keeping alpha channel / transparency the same, if present.
As phrased, this feature would not be a grand color-scheme editor, nor even a new color scheme. It would simply be a checkbox that inverts any existing color scheme to be black-based & distance-from-black-based instead of white-based & distance-from-white-based, so to speak.