Hi Kazuki,
Classes are just class names (selectors) we will use in HTML and CSS. We can specify in mendix the classes you want and we can specify the styles for those classes in vs code. You can refer the style your app with sass learning path in mendix academy for more details. I will attach the link for that learning path here.
https://academy.mendix.com/link/paths/123/Style-your-App-with-Sass
Hope this Helps:)