Resolve "Changes to CSS have broken Dark Theme"

Closes #916 (closed)

Adds purgecss ignore headers to dark CSS to prevent it being purged when production deployment is built

Before

After

front/src/style/themes/_dark.scss

+/* purgecss start ignore */
+
 $background-color: rgb(43, 58, 66);
 $button-hover-color: rgb(33, 48, 56);
 $light-background-color: rgb(51, 71, 82);
@@ -271,3 +273,5 @@ $link-color: rgb(255, 144, 0);
     }
   }
 }
+
+/* purgecss end ignore */