by default wppizza widgets have a classname of wppizza_widget. As this is not referenced in the plugin itself, you could change that classname to something you might prefer.
To do so you can add the following to your wp-config.php

define('WPPIZZA_WIDGET_CSS_CLASS', 'some_classname');

somewhere before
/* That's all, stop editing! Happy blogging. */

where some_classname is the classname you wish to set.