Fullwidth background image opacity/overlay for mobile

Hi,

Is there a way I can make the fullwidth backgound image darker for mobile devices? A dark color overlay will do too.

Please look at the following url:
https://riotphotobooks.com/ksa/

On mobile the text is not so visible on fullwidth sections eg: “Hand Crafted Wedding Albums” and “Sticky Wall Prints”

Thanks,
Fraser