CSS is an excellent addition to plain HTML.
With plain HTML you define the colors and sizes of text and tables throughout your pages. If you want to change a certain element you will therefore have to work your way through the document and change it.
With CSS you define the colors and sizes in "styles". Then as you write your documents you refer to the styles. Therefore: if you change a certain style it will change the look of your entire site.
Another big advantage is that CSS offers much more detailed attributes than plain HTML for defining the look and feel of your site.
Finally, CSS can be written so the user will only need to download it once - in the external style sheet document. When surfing the rest of your site the CSS will be cached on the users computer, and therefore speed up the loading time.
The only reason not to use CSS in your design is not knowing how!
CSS lessons for beginers are given below :