Fix background color of markdown code in dark mode

Make the code highlight in markdown text more
visible when using dark mode.

Changelog: fixed

app/assets/stylesheets/application_dark.scss

@@ -57,4 +57,8 @@ body.gl-dark {
       }
     }
   }
+
+  .md code {
+    background-color: $gray-200;
+  }
 }