Additional CSS in WordPress 4.7 is added with the purpose of providing users an access to better theme customization via custom CSS. The feature available on WordPress Live Customizer as ‘ Additional CSS’ which will allow users to write custom CSS and make minor styling changes with live previews. Nothing to worry about because the additional CSS feature will not do any harm to your custom CSS in your existing template.
All of our WordPress themes (both free and premium) have this feature as we always focus on making our themes simple and user-friendly.
After you switch your website to WordPress 4.7 and update the theme, the CSS that you have written in the Custom CSS field will be automatically migrated to Additional CSS field in the Customizer menu. And, the Custom CSS box in the theme will disappear so there will be a single live editing CSS field. Thereafter, you have to go to additional CSS field in the Customizer menu whenever you have to make some styling changes.