[TYPO3] Add CSS file outside the template

[A. Neves]

Hello,
 
I'm a beginner using TYPO3, and I've been using the latest releases (4.0.x) to create my latest website.
 
I've created a template using Templavoila, for which everything is setup: the images, the basic CSS file(s), layout, etc. I would like to know how can I include a given CSS file, stored for example in a Storage folder, on a certain page and all its subpages, outside the template structure?
 
I've read and searched all (most?)of typo3's documentation, but I don't seem to get how to do this!.
Thank you in advance!