The most flexible way to edit the theme CSS or to add your own code and components to use SCSS. As mentioned before, there are two files already prepared for your changes - _user-variables.scss and _user.scss.
To configure the theme, e.g. to change colours for some elements, to change font sizes, etc., use _user-variables.scss. Any variable from scss/bootstrap/_variables.scss or scss/modules/_variables.scss can be overridden here with your own value.
To add your own CSS code, declare it in the _user.scss file.
SCSS is automatically compiled if you're running next.js development mode.
If you need to import css files for plugins, you can do it in/src/pages/_app.js using following syntax import 'plugin/plugin.css'.