Hi Roel,
To learn about the CSS I would suggest to refer W3schools, it has tutorial for all the CSS properties and also provide editor for each example where you can try hands-on.
In Mendix there are different ways to add custom CSS.
- In Appearance tab of each widget in Style field you can add your own custom styling, or create a custom class in CSS file and use the class name.
- In the Mendix project directory go to theme folder and in web folder create a new .css file and write your custom css, then in main.scss file import that css file.