With the release of Joomla 3.5 we can now use a custom CSS file to override the styles of the default Protostar template. Using the override method by only adding our custom CSS code in the user.css file means the change will not be overwritten and lost should the template itself be updated. It's also easier to maintain with all customizations in one place.
To use the user.css file in Protostar you must create the file yourself and place it in the correct location.
User.css location in Protostar
The system will import the file if the file exists and is not empty, according to the official pull request. Once you have this file in place you can add customizations to override any styles used in the template or extensions. Some extensions can add CSS after a template renders, so if you're not seeing your changes, you may need to implement the !important command.