It is not a good idea to overwrite your core styles because if you update widgets or modules from app store then there is a possibility that you might loose your changes.
A better solution would be to create your own custom css and sass and leave the core files as is.
You can write your custom styles in the theme\web folder
You can refer to the mendix documentation on custom styling
https://docs.mendix.com/howto/front-end/customize-styling-new