Trying to Make Image Background Opaque

I am trying to make the image only opaque while keep the rest of the screen normal.

I have already tried a simple usage of opacity (ex: opacity: 0.9;)

My current code looks like this: HTML

<ion-content class='background' no-bounce>

CSS

ion-content.background{
    --background: url(../../assets/main_background4.jpg) 0 0/100% 100% no-repeat;
}

I would like to just dull the image I am using as a background slightly, but currently the entire screen goes opaque no matter what I try. Is it possible to set opacity some way in the --background part?