Ensure that highlighted code blocks never get a text-shadow regardless of...

Ensure that highlighted code blocks never get a text-shadow regardless of where they are or what theme is used.

app/assets/stylesheets/common.scss

@@ -129,6 +129,10 @@ p.time {
   }
 }
 
+.highlight {
+  text-shadow: none;
+}
+
 .highlight_word {
   border-bottom: 2px solid #F90;
 }