Why ::before in css class doesn't working?

Question

I have this code in PUG...

.container.marginTop.marginBottom.fondoAICE
.form-col
  h5
    | Paragraph

And my CSS is

.fondoAICE{
    position: relative;
}

.fondoAICE::before {
    content: '';
    display: block;
    background-image: url('../img/tree_pattern.jpeg');
    background-repeat: no-repeat;
    opacity: .6;
}

What i trying to do is inserting an image as background with opacity without affecting my text, but ::before seems like doesn't working.