Add background color to pre, too.

app/assets/stylesheets/highlight/dark.scss

@@ -3,6 +3,7 @@
   background-color: #333;
 
   pre {
+    background-color: #333;
     color: #eee;
   }
 

app/assets/stylesheets/highlight/monokai.scss

@@ -12,6 +12,7 @@ $monokai-purple: #ae81ff;
   background-color: #272822;
 
   pre {
+    background-color: #272822;
     color: $monokai-fg;
   }
 

app/assets/stylesheets/highlight/solarized_dark.scss

@@ -3,6 +3,7 @@
   background-color: #002B36;
   
   pre {
+    background-color: #002B36;
     color: #eee;
   }
 

app/assets/stylesheets/highlight/white.scss

@@ -3,6 +3,7 @@
   background-color: #fff;
   
   pre {
+    background-color: #fff;
     color: #333;
   }