Issue 3941: Code preview dark doesn't display correctly with long lines

Assigning the background color to the pre tag did not work correctly. Moved background color to the highlight class which fixes the problem.

app/assets/stylesheets/highlight/dark.scss

 .black .highlight {
+
+  background-color: #333;
+
   pre {
-    background-color: #333;
     color: #eee;
   }

app/assets/stylesheets/highlight/monokai.scss

@@ -8,8 +8,10 @@ $monokai-dark: #3b3a32;
 $monokai-purple: #ae81ff;
 .monokai .highlight {
+
+  background-color: #272822;
+
   pre {
-    background-color: #272822;
     color: $monokai-fg;
   }

app/assets/stylesheets/highlight/solarized_dark.scss

 .solarized-dark .highlight {
+  
+  background-color: #002B36;
+  
   pre {
-    background-color: #002B36;
     color: #eee;
   }

app/assets/stylesheets/highlight/white.scss

 .white .highlight {
+  
+  background-color: #fff;
+  
   pre {
-    background-color: #fff;
     color: #333;
   }