New colors, new buttons

Colors removed:
$wds-fandom-color-mint
$wds-fandom-color-pine
$wds-fandom-color-plum
$wds-fandom-color-purple

Colors renamed:
$wds-fandom-color-warm-gray -> $wds-fandom-color-beige

New color:
$wds-fandom-color-button-background

Added light version of secondary button (to be used on dark backgrounds)