Gist para cambiar el Login de WordPress

functions.php

function admin_styles() {
  wp_enqueue_style( 'vegasCSS', get_template_directory_uri() . '/login/css/vegas.min.css', false );
  wp_enqueue_style( 'loginCSS', get_template_directory_uri() . '/login/css/loginStyles.css', false );

  wp_enqueue_script( 'jquery' );
  wp_enqueue_script( 'vegasJS', get_template_directory_uri() . '/login/js/vegas.min.js', array('jquery'), '1.0.0', true);
  wp_enqueue_script( 'loginjs', get_template_directory_uri() . '/login/js/login.js', array('jquery'), '1.0.0', true);


  wp_localize_script(
    'loginjs',
    'login_imagenes',
    array(
      "ruta_plantilla" => get_template_directory_uri()
    )
  );
}
add_action('login_enqueue_scripts', 'admin_styles', 10 );

login.js

jQuery(function($) {
    $('#login h1, #login form').wrapAll('<div class="grupo"></div>');

    $("body").vegas({
      slides: [
        { src: login_imagenes.ruta_plantilla + "/login/img/1.jpg" },
        { src: login_imagenes.ruta_plantilla + "/login/img/2.jpg" },
        { src: login_imagenes.ruta_plantilla + "/login/img/3.jpg" }
      ],
      overlay: login_imagenes.ruta_plantilla + "/login/img/overlays/05.png",
      transition: ['fade', 'zoomOut', 'swirlLeft']
    });
});

loginStyles.css

body.login {
  background: transparent;
  padding-top: 100px;
  box-sizing: border-box;
}
body.login div#login {
  background-color: rgba(0,0,0, .65);
  padding: 40px 30px;
  border-radius: 15px;
  -webkit-box-shadow: -3px 7px 35px 1px rgba(0,0,0,.8);
  box-shadow: -3px 7px 35px 1px rgba(0,0,0,.8);
}
@media screen and (min-width:768px) {
  body.login div#login {
    width:700px;
    margin:auto;
    position: absolute;
    top:0; left:0; bottom:0; right:0;
    height: 330px;
  }
}
body.login div#login h1 {

}
@media screen and (min-width:768px) {
  body.login div#login h1 {
      display: inline-block;
      margin-top: 110px;
      width:50%;
  }

}
body.login div#login h1 a {
  background-image: url(../img/logo.png);
  height:100px;
  background-size: 50%;
  width:100%;
  margin:0;
}
body.login div#login form#loginform {
  float: right;
}
body.login div#login form#loginform p {
  color: #000000;
}
body.login div#login form#loginform p label {
  color: #000000;
}
body.login div#login form#loginform input {
  background: #ffffff;
  border:1px solid #000000;
}
body.login div#login form#loginform input#user_login {}
body.login div#login form#loginform input#user_pass {}
body.login div#login form#loginform p.forgetmenot {
  margin-top: 7px;
}
body.login div#login form#loginform p.forgetmenot input#rememberme {}
body.login div#login form#loginform p.submit {}
body.login div#login form#loginform p.submit input#wp-submit {
  background-color: #000000;
  border:none;
  text-transform: uppercase;
  padding: 5px 40px;
  height:auto;
  box-shadow: none;
  text-shadow: none;
  font-weight: bold;
}
body.login div#login form#loginform p.submit input#wp-submit:focus {
  box-shadow: none;
}
body.login div#login p#nav {
  text-align: center;
}
@media screen and (min-width:768px) {
  body.login div#login p#nav {
    display: inline;
  }
}

body.login div#login p#nav a {
  color:#ffffff;
}
body.login div#login p#backtoblog {
  text-align: center;
}
@media screen and (min-width:768px) {
  body.login div#login p#backtoblog {
    display: inline;
    float: right;
    margin:0;
  }
}
body.login div#login p#backtoblog a {
  color: #ffffff;
}

#login_error {
  width:100%;
  position: absolute;
  top:0;
  left:0;
  box-sizing: border-box;
}

.grupo {
  margin-bottom: 20px;
}
.grupo:after {
  visibility: hidden;
  display: block;
  content:"";
  clear: both;
  height: 0;
}

La solución del formulario fuera de la caja, para corregirlo es agregarle un @media screen y margin-top: 183px;

acá está...

body.login div#login form#loginform { float: right; } @media screen and (min-width:768px) { body.login div#login form#loginform { float: right; margin-top: -183px; }