For custom CSS/SCSS, you should create new files in the theme folder. Here is link with complete details and steps on setting up custom CSS/SCSS
https://docs.mendix.com/howto/front-end/customize-styling-new
To add to what Umar said – I would highly recommend using an editor that allows you to open an entire folder, such as Visual Studio Code. That way you can easily just do an automatic search through all files for the class name, and you can see how the scss was originally structured (before you make your own custom files).