Small feature request -- Ability to change close "X" on promo blocks to light or dark

We've had some issues with folks being unable to see the close button in the interstitial promo blocks. The problem is it's impossible to see on the block if it's black in front of an image, or if it's white against a default white background. It can be changed manually via css, but this would need to be changed anytime the layout of a promo block is changed. By adding the "light/dark" option to the close button text in the block options it would make it a quick adjustment that could be done via the admin panel. 

 1) What problem(s) does this idea solve? Why do you need this idea implemented? Provide as many problems or use cases as possible. Makes it easier for users to see the button and not got frustrated by being unable to close promo interstitial. Would make it easy for administrators to correct small design oversights. . 

 2) How often would you use this feature? All the time

3) How many people in your organization would use this feature? Every site manager and web staff -- appx 12-20.