You cannot upload your own CSS for your e-Portfolio, however, users who are comfortable editing CSS code may choose to customize the look of their e-Portfolio by clicking "Customize" in the Choose a Theme area of their Settings page, editing their CSS code and then saving their changes. The Settings page is accessed from the "Portfolio Tools" drop-down menu. Don't worry, you can always click the "Reset" button if you wish to return to the default CSS code!
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