What is the optimal place to store CSS files?
In most cases, the best place to store CSS is in a publicly accessible directly, not managed by Craft at all. Such as:

public/assets/css/master.css


Storing your CSS and other front-end assets outside of Craft reduces the overhead of accessing those files. While Craft allows you to store css files in your Craft templates folder, there is additional overhead to access those files as they need to be processed by Craft.