The problem is exactly as you say, Mosiac, Plone 5 and Diazo - this is too many layers of abstraction just to get a theme usable.
As far as integrating a custom theme from themeforest.com say, It's actually relatively simple to use Diazo and a rules.xml file to create some conditions. The problem therein is that you lose (in my opinion) a lot of flexibility that working directly with Plone allows. This is why using just defaults would really help this situation.