Custom CSS edit on a wordpress theme

I need some custom CSS writing for a wordpress theme, to replicate the blurred and darkened background of the posts on this customised theme:

http://preview.themeforest.net/item/share-it-timeline-wordpress-theme/full_screen_preview/10833644?_ga=2.190457132.660157020.1611307302-1637046597.1600265090

Here is our current implementation:

http://18.133.158.117/

The change needs to be applied on the sticky post and we also need to be able to choose to use it on other posts. It is implemented by using a separate image as the post background, and managing this image so it "sticks" to the image of the actual site background. Something like in this link - https://webdesign.tutsplus.com/tutorials/how-to-create-a-frosted-glass-effect-in-css--cms-32535

Dev access to our site will be provided, and blurred image