In settings > privacy there is the code for the “Piwik opt-out for your visitors”, which has to be embedded in form of an iframe. In order to change the font-family etc. of the text you have to modify the file lang/de.php. But this modification will be canceled on every PIWIK-update. This is not very comfortable. That ist why a configuration-possibility for this opt-out-text would be my wish for one of the next releases.