You should look at how bootstrap structures their css. It's a good design to follow. On a general level:
1) you have one high-level class for UI -types (like .alert, .carousel, .btn, etc).
2) You modify that class with additional classes like .alert-success or .btn.active

You can also look at how they structure their SASS. Generally SASS should be structured by 'ui' - never by page so that you can keep things reusable and consistent.