add commentary to colour codes

since we're keeping the original names of colour variables from the
theme, it's a good idea to at least have the usage of them (somewhat)
documented.

assets/scss/_predefined.scss

@@ -1,11 +1,12 @@
 // Colors
 //
-$theme: #613583;
-$text: #21fA45;
+$theme: #613583; // hyperlink highlight background colour, .post-info
+                 // site-local link :hover highlight colour
+$text: #21fa45; // colour of normal text
 $light-grey: #010000; // background
 $dark-grey: #111115; // hover bar
-$highlight-grey: #9141ac; // highlight background of `keyword`
-$midnightblue: #151718; // code background, link background
+$highlight-grey: #9141ac; // code (not in pre) background
+$midnightblue: #151718; // code text (not in pre) colour
 
 // Fonts
 //