In the CustomGoogleSearch class you have: $this->plugin_url

In the CustomGoogleSearch class you have:

$this->plugin_url = WP_PLUGIN_DIR;

which does not account for if the page is being served over HTTPS, so your CSS files do not download on most browsers if viewing over HTTPS.

The correct method of doing this is:

$this->plugin_url = plugin_dir_url(__FILE__);

For more details see: http://wordpress.org/ideas/topic/wp_plugin_url-doesnt-take-ssl-into-account