The plugin uses the default wp_enqueue_styles() method to include the styles, which uses the plugin_dir_url(__FILE__) core wordpress function. This returns the URLs as set on the site settings page.
In case you recently transitioned from simple http to ssl, make sure to change the site URL on the back-end, in the wp-config.php file as well as clearing all layers of caches. Unfortunately I'm not aware of any kind of https related issues, as the plugin itself is developed and tested on a SSL enabled environment. Usually the error is the cache or the incorrect site url option, I would recommend checking those first.
There are also two very handy methods to ensure site URL change as described in the wordpress codex: https://goo.gl/8CZuNF
I usually use these constants in the wp-config.php to prevent any mistakes:
If you like my products, don't forget to rate them on codecanyon :)