I am Using Hex Themes to use Custom Themes on My A6+ And its great with alot of modification.
Its available on Google Play-
Its a paid application and no crack is available on Internet But One of my friend craked it and it worked.
Using It we can easily build themes for ONE UI from scratch.
It support many third party apps like Instragam,Whatsapp,YouTube,Maps,Gmail etc.
Also prebuilt themes are available