Please try re-adding the background color to the promo popup then clear cache and check back.
And make sure that the MC4WP plugin and the form exist that you added the shortcode.
If the problem continues then deactivate all the third-party plugins and provide admin panel login details of the website to check it myself and help you out accordingly.
Your issue has been solved. Please check the site now after removing the browser cache. It was your site cache, I have deleted the cache from your site wp-content folder now the color is showing fine.