From Facebook Developer Wiki

CSS, Design, and the DOM

Keep in mind the following pointers when styling your application:

• style attributes are allowed, though url() tokens cannot reference JavaScript.
• You can reference your own external stylesheets -- see Include files for details. Or you can use style tags. Facebook will automatically rewrite CSS and prefix every rule with a selector so that the rules specified by the parsed FBML will only affect the application and not the overall appearance of Facebook.
• id attributes get prefixed with the string app followed by the application ID of the calling application, followed by an underscore. So, this FBML:

<div id="mybox"></div>

would become:

<div id="app2235437893_mybox"></div>

Additional Information

• For tips and tricks regarding CSS usage, see CSS Tips and Tricks.
• To see some styles that Facebook uses, see Facebook Styles.

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