Different css of .gform_wrapper for every form

Can I use different wrapper for every form? I was trying to use .gform_wrapper_7 for form ID=7 but it doesn't work :(

.gform_wrapper {
    background-color: #EF8DB4;
    border: 4px solid #E6BFCD;
    -moz-border-radius: 10px;
    -webkit-border-radius: 10px;
    border-radius: 10px;
    color: black;
    float: left;
    margin: 25px 40px;
    overflow: inherit;
    padding: 10px;
}
.gform_wrapper_7 {
    background-color: #000000;
    border: 4px solid #FFFFFF;
    margin: 25px 40px;
}

What am I doing wrong? It always shows default .gform_wrapper and NOT .gform_wrapper_7