CSS is just one component in Web Design. It is important to learn especially if you are planning on Designing AND Developing websites for yourself or for clients. However, there are several other important aspects to learn for web development as well.
If you are working solo — it’s critical to learn CSS, HTML, Javascript, and MySQL along with a CMS like WordPress.

If you are working in a Web Agency environment then you could focus only on design and site structure then a lot of the time you don’t need to code. However, you will have to make sure all the details are handed off to the front end and back end developers for your site. You would also have to oversee the process once it goes into development.

Best of luck!