How can I change the login form’s look in wordpress

Solution:

You can do it without changing the core files, using the hook login_head, simply add these code inside your functions.php file and you are done

function custom_login_logo() {
    echo '<style type="text/css">
        h1 a { background-image:url('.get_bloginfo('template_directory').'/yourImageFolder/login_logo.png) !important; }
    </style>';
}
add_action('login_head', 'custom_login_logo');

Update: Also to change the link of the logo to the current site use following code snippet

function custom_login_url() {
    return get_bloginfo( 'siteurl' );
}
add_filter( 'login_headerurl', 'custom_login_url' );