CSS

media query css file

<style>
    @media only screen and (max-width: 600px) {
        /* CSS rules for browser widths equal to or less than 600px */
        body { background-color: #ffffff; }
    }
    @media only screen and (min-width: 601px) and (max-width: 1200px) {
        /* CSS rules for browser widths from 601px to 1200 px */
        body { background-color: #ff0000; }
    }
    @media only screen and (min-width: 1201px) {
        /* CSS rules for browser widths from 1201px and up */
        body { background-color: #0000ff; }
    }
</style>