first fixes to dark variant. not sure about the conditionals

- I do want to limit the number of custom colors,
but perhaps instead of using conditionals we could use
'derived' colors for the two variants. And keep the high level
palette minimal ("API").

$theme_bg_color // High Level Color API

$_highlight = lighten($theme_bg_color, 50%) //light
$_highlight = transparentize(lighten($theme_fg_color,30%),0.9) //dark