To remove the background make this adjustment. You can also choose to just change the 4th rgba value to  "0"#prompt-background {background: none; url("{{ 'age-check-background.jpg' | asset_url }}") no-repeat center center fixed rgba(0,0,0,0.5);-webki...

Sounds like you need responsive css styling on your modal/pop-up so that it renders correctly on mobile.