There is a bug in this plugin, causing it to return http:// instead of https:// when the site is accessed securely... This is due to using the get_option('site_url') function, which doesn't work properly.
I fixed it by changing line 205 and 207 in sermon.php to:
wp_register_style('sb_style', trailingslashit(site_url()).'?sb-style&', false, sb_get_option('style_date_modified'));
wp_register_style('sb_style', trailingslashit(site_url()).'sb-style.css', false, sb_get_option('style_date_modified'));