contacts-v2: use light icons in contact details

Author: Yvonne Yip

shiny/Contacts-v2/components/contacts-detail.html

@@ -42,7 +42,7 @@
 			<div class="detailHeader" style="background: url('{{contact.background}}?{{contact.guid}}') repeat-x;">
 				<polymer-flex-layout vertical="true"></polymer-flex-layout>
 				<template if="{{!desktopLayout}}">
-					<polymer-ui-toolbar>
+					<polymer-ui-toolbar theme="polymer-ui-dark-theme">
 						<polymer-ui-icon-button icon="left" on-tap="{{backAction}}"></polymer-ui-icon-button>
 						<div flex></div>
 						<!-- <polymer-ui-icon-button icon="edit"></polymer-ui-icon-button> no such icon -->