As per discussed here: https://make.wordpress.org/themes/2015/02/03/theme-review-chat-notes a need was found to make custom css fields safe for MySQL injection. It was mentioned that Jetpack had this integration built in. This is that Jetpack code isolated and in an example form for any to use.
Redux has integrated this into their default CSS validator. You can learn how to use this validation class in Redux here: http://docs.reduxframework.com/core/the-basics/validation/
Integration is pretty strait forward. Prior to saving your data to the DB (filter/hook), you need to pass it through the code found in validate_css.php. Replace $value with your passed value before saving.
Also a warning will be echo'd on line 44 that you can just use for your own internal messaging.
This code is directly from the Jetpack plugin by WordPress.com and follows their CSS sanitization standards, while at the same time taking out their LESS/SCSS pre-compilers.