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I would like to leave a small space between the navigation bar and the menu (approximately 20pxs) without affecting the background image positioning.
HTML
<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <link rel="stylesheet" type="text/css" href="css/text.css">
        </head>
    <body>
        <div id="header">
            <nav>
                <ul>
                    <li><a href="#">HOME</a></li>
                    <li><a href="#">SOBRE</a></li>
                    <li><a href="#">SERVIÇOS</a></li>
                    <li><a href="#">CONTATO</a></li>
                </ul>
            </nav>
            </div> 
    </body>
</html>
CSS
* {
     margin: 0;
     padding: 0;
    font-family: arial;
}
/** Background image **/ 
#header {
    background-image: url(../img/background.jpg);
    width: 100%;
    height: 550px;
    background-repeat: no-repeat;
    position: relative;
    background-position: center;
}
nav {
    background-color: black;
    opacity: 0.7;
    text-align: right;
}
nav li {
    display: inline-block;
    padding: 25px 0 25px 0;
}
nav a {
    color: white;
    padding: 0 40px 0 40px;
    text-decoration: none;
}

What do you call "navigation bar"?
– Sam