html {
    font-family: Arial;
}
body {
    background-image: url(flash.jpg)
}
header {
    background-color: black;
    opacity: 0.7;
    color: white;
    padding: 2%;
    font-size: 200%;
    border-radius: 25px;
}
ul {
    opacity: 0.7;
    list-style-type: none;
    padding: 0;
    margin: 0;
    overflow: hidden;
    background-color: black;
    border-radius: 25px;
}
li {
    float: left;
}
li a {
    text-decoration: none;
    color: white;
    display: block;
    text-align: center;
    padding: 14px 16px;
}
li a:hover {
    background-color: gray;
}