Use native textarea when editing user rules

Use native textarea (no color set) when editing temporary user rules.
Improves visibility significantly when eg. using Firefox with a dark GTK theme.

src/css/user-rules.css

@@ -139,7 +139,6 @@ body[dir="rtl"] #commitButton:before {
     text-decoration: line-through;
     }
 #diff textarea {
-    background-color: #f8f8ff;
     border: 0;
     border-top: 1px solid #eee;
     direction: ltr;