Definitely CSS file.
CSS is meant to be written freely and should have it's own place. In a css file you can see all the scoping you need (SCSS (Sass) is even better by the way, very suggested!), overrides of such and etc.
In addition, something you should keep in mind is CSS overrides - bad for performance.
With a separate CSS file you use class and id, and have such a better workspace for writing editing and debugging CSS.
Plus, inline css will override almost any other css rule you give the specific element - little dangerous.