Some people like to use more than one stylesheet on their website. It may be for valid reasons, like presenting different styles for different media types or user-agent, or settings different styles for different pages or different sections of pages. While there is nothing to prevent you from adding as many stylesheet you want the key is not to overuse it. Limiting the number of connections the browser makes to server is necessary and combining stylesheets is one good practice.
CSS Mixer is a small application that will help you combine multiple stylesheet into one. It can also compress the size of the stylesheet by removing duplicate declarations and unnecessary elements like blank lines and spaces, and using shorthand method of declaring styles.
Just load the folder containing the CSS files, select the files you want to combine and click on Minimize.
Also see CSS Toolbox