Assuming You want to do this as a job, start where the majority of potential customers are. https://codex.wordpress.org/Theme_Development https://codex.wordpre

Assuming You want to do this as a job, start where the majority of potential customers are. https://codex.wordpress.org/Theme_Development https://codex.wordpress.org/Theme_Development There are tons of tutorials out there, intuition correlates with experience in most cases.