I suggest you dig into the Intermediate path through this module:
https://academy.mendix.com/link/paths/135/Implement-a-Design-System and also this one
https://academy.mendix.com/link/modules/482/lectures/3874/-5.4.1-Changing-Header-Styles
It explains how to set partials and use them in your project
If you only modify .scss files Mendix will show the changes but you risk losing the changes if a component or Studio receive an update.