How to get current screen width in CSS?

I use the following CSS code for formatting when screen width is less than 480px, and it works well.

@media screen and (min-width: 480px) {
    body {
        background-color: lightgreen;
    }
}

I would like to get the current width for calculation to use zoom like it follows:

@media screen and (min-width: 480px) {
    body {
        background-color: lightgreen;
        zoom: (current screen width)/(480);
    }
}