Add support for user-selectable dark/light theme

Updates Settings to use the DayNight theme (real name TBD) and fixes
several colors that were pinned to non-theme values. Adds a setting for
switching theme between light/dark/auto.

Bug: 19368658
Bug: 19369392
Change-Id: I7cd23b7ad38bdb25a01b92e67ff1ee1cdd8a3958