In Studio Pro, the field to apply a CSS class is a free-text. It would help (even beginners) if it could guide us to some available classes.
Just auto-suggestion would be great, eventually a search popup.
When you explore the CSS files, you realize that a lot of styling already exist and worth to be tested
tbhm you should really try to avoid the css field in mendix.
there are some realyl great guides how to start styling in mendix with sass, and most of the time you would be using a IDE, that has auto complete.