Update editor switcher icons and fix CSS

Update icons as per Kaity's request. Also update CSS:

* use icon-32px class which is not deprecated
* remove icon-40px class (not used anymore)
* change content padding on tablets to 3.35em which is the size of a
  button in overlays

Change-Id: I9c8e38b83bd620dbe2085892fb8de9edccc59c69
Merge "Update editor switcher icons and fix CSS"