Normally when someone wants to develop a web site, they must know how to use CSS to it's fullest potential, since CSS is what makes your site look great.
CSS is mostly a set of commands, moulding your site into the design you want. CSS was introduces to make web styling a lot easier, and that's exactly what it is good at.
Here's a example:
I just basically designed a block to have the name 'main' and have a white background with the size of 500 pixels by 500 pixels.
CSS can be added into a html file quite easily, from attaching a seperate CSS file normally called, style.css, you can also do the CSS styling within a html document like this.
Learning and using CSS is fun and each day is different, you learn something new everyday since it's a technology that keeps on growing.
Have a look at this article about how to write better CSS