CSS - How do I effect this div in this site [closed]

Here's what I wanted

* {margin:0; padding: 0;}
html, body {
    height:100%;
}
.header {
    width:100%; 
    height:auto; 
    min-height:100%; 
    /* 
    aqui é onde acontece a mágica, 
    a propriedade height:auto e min-height: 100% 
    fará com que esta div ocupe sempre a altura total 
    do navegador, mesmo quando redimensionado 
    */
    background:cornflowerblue;
}

Credits: link

Create a <div> , for example <div class="barra-topo"> with content: background, buttons, fields, and so on. In CSS of the background use widht: 100%; for the background to fill the entire screen.

You will have to create the viewports so that they change according to the device. Notice that the bar model is the same, but the content changes. Search for CSS media queries.