SpacellaGaming there's no official tutorial yet.
If you write a CSS theme you should be able to get started with knowledge of CSS and the browser inspect tool.
If you want to alter the structure of components then it's the same as writing Flarum frontend extensions, there is a short introduction here https://flarum.org/docs/extend/frontend.html#changing-the-ui
To learn more, the best at this point is to take a look at the many existing open source extensions. Other people also shared themes you can take a look at.
