Please check the docs:
https://docs.mendix.com/howto/front-end/create-a-company-design-system/
What I'd recommend is making a UI resources module in which you make a call to the Material UI css / import it, then make sure you change your app settings (settings -> theme -> make your own module the highest). After that you should be able to use the classes from whichever (css-)framework you have imported. But do realise that anything that is not specifically stated in that framework will be overwritten by the default Atlas styling!