I understand that in Firefox OS every app is a web app, but I'd like to create a "native" look & feel in my own application, meaning that I want it to "fit in" with the built-in application styles.
Mozilla even has a style guide for this:
http://www.mozilla.org/en-US/styleguide/products/firefox-os/
Is there a stylesheet and/or a JavaScript which I can include in my app in order to create controls and UI elements that look like the ones in the style guide?
(The only download I could find on that site was a PSD containing all the designs, but I'm looking for something ready-to-use.)
You have the good link for some UI guidelines, but you can check what we call the Building Blocks or even get those on GitHub. Another way is to see what we did in GAIA, but in any situations, it's not just about importing a CSS file.