As far as I know, you have to change the colours.css file in your template.
For prosilver, this can be found at /styles/prosilver/themes. In colours.css, search for "/* Post poll styles" and then modify whatever colours you need. Just remember that you have to refresh the theme in the Admin Control Panel. (ACP > Styles > Themes and then hit 'Refresh' next to the theme you're currently using).
Hope that helps.