How to create the first theme on diafan.cms?

Question

CMS

Good day. I'm new and just recently I had a need to create a personalized theme. Google gives out old information from 2012-2016, during this time cms has changed a lot. Tell me, please, what should I start from in order to create a theme and install it correctly on the engine. I would be very grateful if you write down a step-by-step analysis from layout to installing a theme on the engine. Thanks