Use correct theme for the off icon of NightLight, fix tooltip and a11y

ash/ash_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!--
 This file contains the strings for ash.
 -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       output_all_resource_defines="false" source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/ash_strings.h" type="rc_header">
@@ skipping 201 matching lines @@
       <message name="IDS_ASH_STATUS_TRAY_MESSAGE_CANNOT_ADD_USER" desc="The error message when the user has reached the limit of multi profile users.">
         You can only have up to three accounts in multiple sign-in.
       </message>
       <message name="IDS_ASH_STATUS_TRAY_MESSAGE_NOT_ALLOWED_PRIMARY_USER" desc="The error message when the primary user forbids multiple signin because of not-allowed policy or because of usage policy-pushed certificates.">
         The administrator for this account has disallowed multiple sign-in.
       </message>
       <message name="IDS_ASH_STATUS_TRAY_MESSAGE_OUT_OF_USERS" desc="The error message when all the users are added into multi-profile session.">
         All available users have already been added to this session.
       </message>
       <message name="IDS_ASH_STATUS_TRAY_NIGHT_LIGHT" desc="The label used for the button in the status tray to toggle the Night Light feature (which controls the color temperature of the screen) on or off.">
-        Toggle Night Light
+        Night light
       </message>
       <message name="IDS_ASH_STATUS_TRAY_CAST" desc="The label used as the header in the cast popup.">
         Cast screen
       </message>
       <message name="IDS_ASH_STATUS_TRAY_CAST_CAST_DESKTOP" desc="The label used in the tray popup to tell the user we are casting the desktop.">
         Casting screen to <ph name="RECEIVER_NAME">$1<ex>Living Room</ex></ph>
       </message>
       <message name="IDS_ASH_STATUS_TRAY_CAST_CAST_DESKTOP_ACCESSIBILITY_STOP" desc="Stop button accessibility label used in the tray popup to tell the user to stop a cast to the desktop.">
         Stop casting screen to <ph name="RECEIVER_NAME">$1<ex>Living Room</ex></ph>
       </message>
@@ skipping 810 matching lines @@
 
       <!-- Tray scale strings -->
       <message name="IDS_ASH_STATUS_TRAY_SCALE" desc="The label used in scale setting detailed page of ash tray popup.">
         Display scale settings
       </message>
       <message name="IDS_ASH_STATUS_TRAY_SCALE_SLIDER" desc="The accessible text for the scale slider">Display scale</message>
     </messages>
   </release>
 </grit>