Ri Xu Online

First in your theme functions.php file add the following code

// Modify WordPress Dashboard login page
function login_enqueue_scripts(){
    echo '

        '."\n";
    }
add_action( 'login_enqueue_scripts', 'login_enqueue_scripts' );

Then, according to the above code shows that you want to create a custom-login.css file, and then into the root directory of your own theme.

.background-cover{
    background:url(./img/loginbj.jpg) no-repeat center center fixed; 
    -webkit-background-size: cover; 
    -moz-background-size: cover; 
    -o-background-size: cover; 
    background-size: cover; 
    position:fixed; 
    top:0; 
    left:0; 
    z-index:10; 
    overflow: hidden; 
    width: 100%; 
    height:100%;
} 
#login{
    z-index:9999;
    position:relative;
}
.login form { 
    box-shadow: 0px 0px 0px 0px !important;
}
.login h1 a {
    background-image:url(./img/login-logo.png) !important;
    background-size: auto !important;height:52px !important; 
} 
.login #nav a, .login #backtoblog a {
    color:#fff !important;
    text-shadow: none !important;
}
.login #nav a:hover, .login #backtoblog a:hover{
    color:#f99009 !important;
    text-shadow: none !important;
}
.login #nav, .login #backtoblog{
    text-shadow: none !important;
}
.login #backtoblog a {
    color:#fff;
}
#backtoblog {   
    border-bottom:0;
    height:30px;
    left:-520px;
    position:absolute;
    top:0;
    width:100%;
}

Next, we just pictures according to their own path, modifications.