Hello,
I had the same issues. My situation was similar. Although I have an SSL with a content delivery network CloudFlare. I dove into the php of the WP Add Custom CSS plugin. If you are able to do this, then this fix worked for me.
Open the file in the plugin folder called “wordpress-add-custom-css.php” this should be in the main folder.
Search for this function (It starts on line 290 and ends at 291)
wp_register_style( 'wp-add-custom-css', $css_base_url . '?display_custom_css=css' );
wp_enqueue_style( 'wp-add-custom-css' );
and then change the value “$css_base_url” to your full base URL like this
wp_register_style( 'wp-add-custom-css', 'https://www.sample.com/' . '?display_custom_css=css' );
wp_enqueue_style( 'wp-add-custom-css' );
This worked for me.
keep in mind if this plugin ever updates this code will be updated again. This worked for me April 30, 2018.