The usual definition of FSD: MEAN + WAMP. Or similar/other platform equivalent.
MEAN = MongoDB, Express.js, Angular, and Node.js.
WAMP = Windows, Apache, MySQL, PHP.
All of these include various things. But if CSS is the only thing you don't do then I wouldn't worry about it. Just be aware of the technology out there associated with CSS and at least have a basic grasp of what it does you'll be fine.