.transparency{
background-color: rgba(255, 255, 0, 0.3); /* <-- this line is modified */
display: block;
margin: 0 auto;
width: 300px;
height: 500px;
}
.content{
color: #000;
text-align: center;
}
img{
/* Set rules to fill background */
min-height: 100%;
min-width: 1920px;
/* Set up proportionate scaling */
width: 100%;
height: auto;
/* Set up positioning */
position: fixed;
top: 0;
left: 0;
z-index:-1; /* <-- this line is added */
}