Darken/lighten a hex colour by the given amount Stolen from https://benjaminlmoore.wordpress.com/2014/03/18/guardian-data-blog-uk-elections/
darken_colour(hex_colour, amount = 0.15)lighten_colour(hex_colour, amount = 0.15)
strings e.g. "#345678"
fraction to darken by. default 0.15
darkened hex colours