It should be possible to apply a css file to the html document created by CoreAdminHome.optOut. The recommended way to include the optout into a website is to include it via iframe, which lacks the possibility of styling the created output.
In view of maximal flexibility there should be the possibility to pass an URL to an external css file via GET-Parameter. Also there should be a class applied to the body tag, so that an external css file could handle the styling.
I made a quick hack in my piwik installation, a patch file is applied to this feature request.
Thanks. This is already in the queue as ticket #1929. We previously had styling when this was in a separate plugin. If you're interested, please refer to that ticket as there are some security considerations missing from your patch.