Hello Rakesh

Please explain in details. Why do think this would be handy to add extra Custom CSS box? You can add custom CSS to the addon or the page.

Best regards

The custom CSS is not loaded within the .white-popup-block or .mfp-container

By adding this box, you can add custom css for within that modal container.

You can try this CSS on the addon settings. This way you can write CSS for the addon.

#addonId .white-popup-block{
  /* your css */
}

Doesn't seem to work

It works on my end. Anyway, this is the way to add Custom CSS. No more custom CSS box will be added. Apology for the inconvenience.