Frankly, you should have looked at the source code for the material-ui project and the components there. They're using the theme system and reusing or inheriting from there... It's actually really easy to create your own components from the base components outlined. There are also tools for creating customized themes and colors, etc.
It's really in a similar vein to when I talk about Bootstrap, as it's also really easy to look at and use/modify the styling from the source scss files. Likewise you can definitely use the primitives, themes, cards, etc in material-ui's library to create more customized components with the events you need for your own reuse.