body {
    height: 100%;
    font-family: sans-serif;
}
* {
    position: relative;
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}


.header {
    padding: 15px;
    width: 100%;
    background: goldenrod;
    transition: all 0.3s;
}
.header.now-sticky {
    padding: 7px 15px;
}

.content {
    min-height: 1500px;
    padding: 100px;
}
.sidebar {
    float: left;
    width: 250px;
    padding: 20px;
    background: #ccc;
}
.box {
    margin-top: 100px;
    background: red;
    width: 300px;
    height: 300px;
}

.footer {
    min-height: 250px;
    background: #1b1b1b;
}