Please remove these classes from the bootbox header - they should be controlled by the theme otherwise dark themes, such as 'darkly' end up like this:
It seems the style option just needs to be added to the UI (I have been using inline-flow) to choose between white and dark (or primary, success etc) theme for the alert. There is also a mistake in the Modbox code (bootstrap-modbox.min.js) for the modal close button in the header. When using a 'dark' theme the button class should be btn-close-light (it is btn-close-white in the existing code)
Post title updated to reflect this