WordPress - Customizing Login, Register and Lost Password pages

custom-login-styles.css

/*
*
* This CSS template will customize WordPress.org login pages:
* 
* Login page:           /wp-login.php
* Register page:        /wp-login.php?action=register
* Lost Password page:   /wp-login.php?action=lostpassword
*
* @site        https://tudoparawp.com.br 
* @author      Guga Alves
* @twitter     @gugaalves (Feel free to ping me there, feedbacks and greetings are welcome)
* @url         Originally shared on Guga's GIST: https://gist.github.com/gugaalves
* @version     1.0
*
----------------------------------------------------------------
>>> TABLE OF CONTENTS:
----------------------------------------------------------------
        1. GENERAL
        2. LOGIN PAGE
        3. REGISTER PAGE
        4. LOST PASSWORD PAGE
----------------------------------------------------------------*/
/* 1. General
*
*   Those changes will be applied to all wp-login pages
*
*   Image settings */
    body.login {
        background-repeat: no-repeat;
        background-attachment: fixed;
        background-size: cover;
    }
    /* Change font colors*/
    .login form,
    .login #login_error,
    .login .message,
    .login .success,
    .login #backtoblog a,
    .login #nav a,
    .login label,
    #login form p {
        color:#000;
    }
    /* Replace WP logo with your own logo */
    .login h1 a {
        background-image: url('your-logo-file-here.png');
    }
/* 2. Login page
*
*   Customizing Login page only
*
*   Change background - Background position should be changed depending on which image you'll use */
    body.login-action-login {
        background-image: url('/img/login-page.jpg');
        background-position: center center;
    }
    /* Change margins to better position with the image used on that page */
    .login-action-login #login {
        margin: auto auto 10% auto;
    }
    /* Opacity on every form for better visualisation of your stunning background image */
    .login-action-login form,
    .login-action-login #login_error,
    .login-action-login .message,
    .login-action-login .success {
        background-color: rgba(0,0,0,0.1);
        box-shadow: none;
    }
/* 3. Register page 
*
*   Customizing Registration page only
*
* Change background - Background position should be changed depending on which image you'll use */
    body.login-action-register {
        background-image: url('/img/register-page.jpg');
        background-position: center right;
    }
    /* Change margins to better position with the image used on that page */
    .login-action-register #login {
        margin: auto 10% auto auto;
    }
    /* Opacity on every form */
    .login-action-register form,
    .login-action-register #login_error,
    .login-action-register .message,
    .login-action-register .success {
        background-color: rgba(0,0,0,0.1);
        box-shadow: none;
    }
    /* Changing WordPress logo to grayscale - better contrast with the image used on that page */
    .login-action-register h1 a {
        filter: grayscale(100%);
        background-image:url('/img/wordpress-logo.png');
    }
/* 4. LOST PASSWORD PAGE
*
*   Customizing Lost password page only
*
*   Note: If you're using WooCommerce, that plugin uses its own lost password page so you may need to not use that part and create your own customization for that Woo page with body tag .woocommerce-lost-password !
*   If you're using some other plugin managing wp-login pages (like Buddypress) you may need to visit each page, check which body classes should be used and create your own code (classes may be different on custom plugins)
*    
*    Change background - Background position should be changed depending on which image you'll use */
    body.login-action-lostpassword {
        background-image: url('/img/lost-password-page.jpg');
        background-position: center right;
    }
    /* Change margins to better position with the image used on that page */
    .login-action-lostpassword #login {
        margin: auto 10% auto auto;
    }
    /* Opacity on every form */
    .login-action-lostpassword form,
    .login-action-lostpassword #login_error,
    .login-action-lostpassword .message,
    .login-action-lostpassword .success {
        background-color: rgba(0,0,0,0.1);
        box-shadow: none;
    }
/*
*
*   That's it, hope you enjoy it.
*   Thoughts, questions, feedbacks? Feel free to ping me on twitter, @gugaalves
*
*/

instructions.txt

STEPS:

1 - If the theme was not developd by you, first create a child theme (How to create a child theme? Read at https://codex.wordpress.org/Child_Themes) or else modifications will be lost when updating your theme.
2 - Add the following CSS file in your theme/child theme (better if you create a folder to add CSS there).
3 - Add the following snippet in your functions.php file:

function tpw_enqueue_custom_admin_style() {
        echo '<link rel="stylesheet" type="text/css" href="' . get_stylesheet_directory_uri() . '/css/admin/custom-login-styles.css" />';
}

add_action('login_head', 'tpw_enqueue_custom_admin_style');

4 - Please note that '/css/custom-login-styles.css' should have the correct path to the CSS file. So, as example, if you have added the CSS file at /theme-name/assets/css/custom-login-styles.css your code should be '/assets/css/custom-login-styles.css'
5 - Save and that's it!

NOTE: Of course, the image urls in your CSS code should also use the correct path to the image files in your theme folder ;)

PIECE OF CAKE, UH?

And you have the oportunity to edit logo link and alt text with this:

function my_login_logo_url() {
return home_url();
}
add_filter( 'login_headerurl', 'my_login_logo_url' );

function my_login_logo_url_title() {
return 'Your Site Name and Info';
}
add_filter( 'login_headertitle', 'my_login_logo_url_title' );
`

Great additions, thanks @duarteteixeira!