I would also highly recommend checking out the Figma templates Mendix has released to get a feeling for designing with Mendix applications. Not only is this crucial in your communication with developers and application stakeholders, but it also provides you with ready to use templates.
If you already have some development skills you could do the crash course: https://academy.mendix.com/link/paths/82/Crash-Course
If that is a bit to much do the rapid course: https://academy.mendix.com/link/path/31/Become-a-Rapid-Developer
Then filter the learning paths on user interface to get those learning paths.
This way you learn at least a bit on how to create pages and use CSS to design those pages.
Regards,
Ronald