Change color of popup background?

Try this css https://github.com/levantoan/WordPress/blob/master/devvn-image-hotspot/change-background-popup-to-white.css

Thank you. Where exactly should I be placing this CSS code?

Appearance -> Customizer -> Custom css

Got it! thank you so much! It Works!!

One more question, what code should I change to change the color myself? to a grey

the #FFF?

yes
#fff => grey or hex color (#ccc)
rgba(255,255,255,1) => grey or hex color (#ccc)
rgb(255, 252, 252) => grey or hex color (#ccc)