lymdul

You can do this by deregistering the plug-in’s css and append your custom css in your style.css

put this code in your theme’s function.php:

add_action( 'wp_print_styles', 'my_deregister_styles', 100 );

function my_deregister_styles() {
	wp_deregister_style( 'contact-form-7' );
}

you can also deregister any javascripts and styles
http://justintadlock.com/archives/2009/08/06/how-to-disable-scripts-and-styles