Open the  theme.css file in your code editor.

Let’s add the following selectors and discuss what each does to affect the page design.

body {
body {
    padding-top: 50px; /* Required padding for .navbar-fixed-top. Remove if using .navbar-static-top. Change if height of navigation changes. */
}

/* Header Image Background - Change the URL below to your image path (example: ../images/background.jpg) */

.main-header {
    height: 400px;
    background: url('http://placehold.it/1920x400') center center no-repeat scroll;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    background-size: cover;
    -o-background-size: cover;
}

/* Customize the text color and shadow color and to optimize text legibility. */

.tagline {
    text-shadow: 0 0 10px #000;
    color: #fff;
}

.img-center {
    margin: 0 auto;
}

footer {
    margin: 50px 0;
}

At this point you should save your files and go check them out in the browser. Feel free to add / edit / hack away at the code used in this example.