JSR 168 specifies some classes (e.g. portlet-font, portlet-font-label) that can be used in portlets. If your portlet use any of these classes, you can configure what SiteVision font each of these classes should be.
The font mapping can be found on the Template mapping tab in the Appearance category for the site properties. (Note that you must tick the "Show CSS style ..." check box to see the JSR 168 names.)