For the longest time you had to root your phone in order to use custom themes on your device. Over the past year or so OEMs have been adding native theme support to their devices. HTC and Samsung have really been leading the way in this area. It looks like Sony will be stepping up to the plate adding Xperia Theme support to Marshmallow builds on their devices. A new concept build for both the Z3 and Z3 compact have already been released featuring both Marshmallow and the theme manager! The new build also improves stability fixes and other bug fixes.
via XperiaBlog