I want to add an opacity background, with a visible text in front of an image. In my case I used flex to display the div elements, and I tried to create a background with opacity and also with text for each div. I know that I should use z-index to create background opacity layer, but in my case I don't know why it doesn't work.
main{
    border: 2px solid red;
    width: 100%;
    height: 100vh;
}
.projects-wrapper{
    border: 2px solid black;
    width: 100%;
    height: 50vh;
    display: flex;
}
.projects-wrapper div{
    border: 2px solid green;
    flex-grow: 1;   
}
/* .projects-wrapper div.text-description{
    border: 2px solid yellow;
    font-family: 'Open Sans', sans-serif;
    font-size: 20px;
    color: white;
    text-transform: uppercase;
}
.projects-wrapper div.text-description span{
    display: block;
} */
.first-row div:first-child{
    background-image: url(../assets/concert-hall.jpg);
    background-size: cover;
    background-position: center;
}
.first-row div:nth-child(2){
    background-image: url(../assets/bedroom-2.jpg);
    background-size: cover;
    background-position: center;
}
.first-row div:last-child{
    background-image: url(../assets/hotel.jpg);
    background-size: cover;
    background-position: center;
} <main>
                    <div class="projects-wrapper first-row">
                        <div class="image-wrapper">
                            <!-- <div class="text-description">
                                Concert hall <span>in New York</span>
                            </div>
                            <div class="text-category">
                                Architecture
                            </div> -->
                        </div>
                        <div class="image-wpapper">
                            <!-- <div class="text-description">
                                Modern bedroom <span>in Gorizia</span>
                            </div>
                            <div class="text-category">
                                Interior
                            </div> -->               
                        </div>
                        <div class="image-wpapper">
                            <!-- <div class="text-description">
                                Modern hotel <span>in London</span>
                            </div>
                            <div class="text-category">
                                Architecture
                            </div> -->                            
                        </div>
                    </div>
         <main> 
     
     
    