The final new feature in Joomla 3.5 makes it easier to modify Protostar. Currently it's impossible to use a custom.css file and you need to hack the core or copy the template.

Now you'll be able to create a file called user.css inside Protostar. Joomla will now automatically check to see if that file exists. This is a safe way to make changes to Protostar with no core hacks.

The original plan was to call the file custom.css, but that wouldn't work because TinyMCE would load the file and apply the CSS styles to your text editor window.