For CSS, the recommended location is The Style Library in the site collection
Note: Use the style library to store style sheets, such as CSS or XSL files. The style sheets in this gallery can be used by this site or any of its sub sites.
By the way, the Style Library has checked in/check out, unlike the Site Asset library.
_layout folder in case you have access to deploy it to the servers.
See more at
The Site Assets folder usually, though they can live in any document library of your choosing.
The Site Assets folder should already have the correct permissions applied.