how to customize wp-admin page using child theme

Question

There are various plugins that allow you to do this. Just search for ‘login’ in the plugin search.

However, if you’d like to do this yourself with code in your child theme you can use the following code taken straight from the WordPress Codex page on Customizing the Login Form:

function my_login_logo() { ?>
    <style type="text/css">
        #login h1 a, .login h1 a {
            background-image: url(<?php echo get_stylesheet_directory_uri(); ?>/images/site-login-logo.png);
        height:65px;
        width:320px;
        background-size: 320px 65px;
        background-repeat: no-repeat;
            padding-bottom: 30px;
        }
    </style>
<?php }
add_action( 'login_enqueue_scripts', 'my_login_logo' );

You can also add your other custom CSS in the code above to change the background colour or image by targeting the <body> HTML tag in your CSS