Darkle could be done directly with CSS.
What you said is correct, in my first post also i have mentioned as i am not sure which extention need to be extended.
If simple CSS can do the job yes why not. Could you please share the CSS for the same
If possible add toggle on and off this CSS feature in fancy box 😍 (well this is two much to expect, just help me with CSS i will add manually not sure how the injection would work in mobile and desktop)