.center {
    display: block;
    margin-left: auto;
    margin-right: auto;

}

body {
    background-image: url("nakayama.gif");
    background-repeat: no-repeat, repeat;
    background-color: #cccccc;
    background-position: center;
    background-size: 100%;
}
h1 {
    text-align: center;
}
h2 {
    text-align: center;
}
h3 {
    text-align: center;
}