Cascading Style Sheets (CSS) is a stylesheet language which use by web developer to describe the presentation of a document written in a markup language. It use for design your pages like color, fonts, layout etc…CSS makes your Website More SEO friendly as Google, Yahoo and MSN love light-weighted Web sites.
Search engines spiders are actually lethargic. They don’t go through the bundles of HTML code to get to the indexed codes. Font tags and tables make HTML code cumbersome, and thus reduce the accuracy of the results. If you use external CSS files to determine the design attributes, the HTML code will be clean and will create better search engine rankings.
For instance, you could easily make the main content of your site to show up above the header or navigation menu in the code of your Web site; this will help to show search engine crawlers the importance of your content.
Here is complete article about CSS to improve your Organic Search Rankings. Link