Before this commit, if the user chose a light color for the text (with
a dark color for the body for example), that color was also used for the
automatic "dark" color, leaving the "light" color to the default $white.
That means that when a light snippet background was set, the automatic
color which was used was the light color the user chose. We want to
invert the system in that case: use the user color as the "light" color
to use over dark backgrounds and leave the "dark" color to the default
$gray-900. This is what this commit does.