css make background visible in text

1.transparency{
2    background-color: rgba(255, 255, 0, 0.3); /* <-- this line is modified */
3    display: block;
4    margin: 0 auto;
5    width: 300px;
6    height: 500px;
7}
8.content{
9    color: #000;
10    text-align: center;
11}
12img{
13    /* Set rules to fill background */
14    min-height: 100%;
15    min-width: 1920px;
16    /* Set up proportionate scaling */
17    width: 100%;
18    height: auto;
19    /* Set up positioning */
20    position: fixed;
21    top: 0;
22    left: 0;  
23    z-index:-1; /* <-- this line is added */
24}
25