How to make your own themes?
Hi @Diome, the best is to start with a theme you like, use theme inheritance and modify the structure piece by piece. The Twig documentation and the themes section of the learn grav site will be a good start for you. You may also have look how other themes are structured and the best source are the themes directly from the Grav Team https://github.com/getgrav/?utf8=✓&query=theme .
The rest is playing around with HTML/CSS/JS , the usual required combination websites are built of.