[FIX] website: handle light colors as body text color

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.

addons/website/static/src/scss/bootstrap_overridden.scss

@@ -2,7 +2,12 @@
 // Color system
 //
 
-$yiq-text-dark: color('text');
+// TODO should take into account the final values of yiq-text-light and
+// yiq-text-dark, not consider it is white and black, but this is ok as a
+// first approximation.
+$-is-light: if(color('text') != null, (luma(color('text')) > 50%), null);
+$yiq-text-light: if($-is-light, color('text'), null);
+$yiq-text-dark: if($-is-light, null, color('text'));
 
 // Body
 //