background image full screen

Hi all,
I just installed the theme ‘Just married’ but I cannot figure out how to make the background image full screen. I have tried several methods and all have failed.
The only place I can do it when I ‘inspect element’ is by adding `element.style {
background-size:100%;}`
but I cannot seem to find a way to implement this in my css style.

The url is : https://sheppadviser.com.au/adviser-web-wedding-celebrant/

I have implemented some coding in the home page, but the styles don’t follow on to other pages. like: https://sheppadviser.com.au/adviser-web-wedding-celebrant/?page_id=69

Does anybody have any suggestions???

Thank you for your help.

David