Change Opacity of Boxed Layout

I am not a complete novice, but I am very far from confidently knowing code. I would love any help and please spell it out for me. Thanks!

I am revamping the look of my website: https://www.lookbetweenthelines.com/ I downloaded the Nikkon theme, I liked how customizable it is. I decided to add the boxed layout option, so there was a background behind my posts and sidebar. However, I really want it to be semi-transparent, so you can see more of my background. I have googled many different ways of doing this and either can’t find the right code or the right place to put it. I have already messed up syntax and got locked out once, so I decided to try a different route. I figured it would go in the style.css area somewhere in here:

}

.site-boxed {
max-width: 1240px;
margin: 0 auto;
box-shadow: 0 0 8px rgba(0, 0, 0, 0.12);
background-color: #FFF;
}

Is this correct placement? If so how do I change the opacity? Thanks!