Changed preferencePane styling to self hosted module.

lib/stylePreferencePane.js

@@ -0,0 +1,33 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+(function(){
+	"use strict";
+	const { Cu } = require('chrome');
+	const { on } = require('sdk/system/events');
+	const self = require('sdk/self');
+	const { AddonManager } = Cu.import("resource://gre/modules/AddonManager.jsm");
+	const { setTimeout } = require("sdk/timers");
+	const { loadSheet } = require("sdk/stylesheet/utils");
+	AddonManager.getAddonByID(self.id, function(addon){
+		on('addon-options-displayed', onAddonOptionsDisplayed, true);
+	});
+
+  function onAddonOptionsDisplayed({ subject: doc, data }) {
+    if (data === self.id) {
+		loadSheet(doc.defaultView, self.data.url("options.css"));
+		
+		// need to wait unttil the simple-prefs are inserted in the DOM
+		setTimeout(function(){
+			// replace empty menuitems with separators
+			[].slice.call(doc.querySelectorAll("menuitem[value='']")).forEach(
+				function(menuitem){
+					var separator = doc.createElement("menuseparator");
+					menuitem.parentNode.replaceChild(separator, menuitem);
+				}
+			);
+		}, 1);
+    }
+  }
+	
+}());