How to use own css of functionality plugin?

You can hook stylesheet_uri which lets you override the URL to the active theme or child theme’s style.css:

function epf_stylesheet_uri() {
    return plugins_url( '/stylesheets/style.css', __FILE__ );
}
add_filter( 'stylesheet_uri', 'epf_stylesheet_uri', 10, 1 );

This will then use your plugin’s style.css instead of the theme’s. But IMO if you want to override the style you should really be changing the theme instead.