Whenever you want to add a custom style to your's site CSS, I generally see the general "solution" to add the CSS to the template itself, eventually add some extra custom PHP inside your template from the templates folder. I dislike this solution, because then you need to remember that your template is actually tainted, and you can't just install the template and your CSS, you again need to fiddle with its internals.
Obviously it can be seen that I can add multiple custom CSSes, but generally I preffer only one. Since the node is appended to the head later that the head element that was outputted by Joomla, I also get my custom CSS last, which helps when it comes to CSS rules priorities, since the last rule wins for the same selector precision.
I hope it helps, and have fun designing your Joomla site.