html, body {
    height: 100%;
}

body {
    font-family: Arial;
    margin: 0;
}

header {
    background: black;
    color: white;
    padding: 15px;
}

.sidebar {
    width: 25%;
    float: left;
    background: lightgray;
    padding: 10px;
    box-sizing: border-box;
}

.content {
    width: 75%;
    float: left;
    padding: 10px;
    box-sizing: border-box;
}

footer {
    clear: both;
    background: black;
    color: white;
    padding: 15px;
}

.main {
    flex: 1;
}