Example WordPress plugin to override CSS

jetpack-override.css

/* CSS overrides goes here */

#some-elem .some-class {
    display: none;
}

override-css.php

<?php
/*
Plugin Name: Override Jetpack CSS
Plugin URI: http://dessibelle.se
Description: Overrides Jetpack CSS by enqueueing additional CSS
Author: Simon Fransson
Version: 1.0
Author URI: http://dessibelle.se/
*/
 
 
function oc_init() {
    wp_enqueue_style( 'oc-style', plugins_url( 'jetpack-override.css', __FILE__ ), array(), '1.0' );
}
add_action( 'init', 'oc_init' );