Hi,
1. When you are in Configuration > Display > CSS, for the "Frontend css file " option you can select what you want, here you can just let it by default and click on the edit icon to edit the currently used file.
2. You can edit code if already existing, just do a Ctrl + F to find what you want, and if it's not present, add your code at the end of the file.
3. To edit the cart, you can do this via CSS, thanks to your browser console you can find the desired classes to apply the css properties, and for more editions, you can edit the view "product / cart".