Move hljs import to theme file

When using CSS variables, the variables should either have a fallback
value assigned or we need to be sure they are defined somewhere.

app/assets/stylesheets/framework/highlight.scss

-@import '../highlight/hljs';
-
 .file-content.code {
   border: 0;
   box-shadow: none;

app/assets/stylesheets/highlight/themes/dark.scss

 /* https://github.com/MozMorris/tomorrow-pygments */
 
 @import '../common';
+@import '../hljs';
 
 /*
 * Dark syntax colors