theme folder structure
The first step you need to take to create a theme is to create a folder with your theme's name inside the folder /typos/themes/
Style and Settings
Every theme needs two files to be valid. So if we assume your theme's name is mytheme, you need the following files:
The css file contains the theme's stylesheet and the json file has the theme's settings. Read more about the theme's json file here.
Your theme needs blocks so you can compose your pages. You can copy blocks from the default or other themes to use them as a base for your own blocks. You can also copy their style from their css file or style them from scratch. Learn the basics about creating blocks here.