Changing the "Big Title" text color opacity in Zerif Lite theme

I would like to change the opacity of the Big Title in the homepage in Zerif Lite theme, but I cannot find the right place for it in style.css. I am already using a child theme.

I believe this shouldn’t be a mystery, but I just cannot deal with it myself.
I would be very grateful if you could help me ??

My webpage is not live yet, so I cannot provide the link.