How to customize the user login form?

Question

I'm trying to customize user login form in /user/login page and I want to use a hook_form_alter function to add some awesome style and placeholder to each of user login form fields and remove or hide:
"Enter your admin username" and "Enter the password that accompanies your username" description expressions under each of theme.
To resolve this issue I added something like below code to my ThemeName.theme:

function ThemeName_form_alter(&$form, $form_state, $form_id) {
  if ( $form['#form_id'] == 'user_login_form' ) {
    $form['actions']['submit']['#attributes']['class'][] = 'button glow button-primary expanded';
    $form['keys']['#attributes']['placeholder']['name'] = t('User name');
    $form['keys']['#attributes']['placeholder']['password'] = t('Password');
  }
}

I try out many different ways but I can't get any result. in finally I want to achieve something like below image:

I read this pages but seems none work in Drupal 8:
How to modify the fields of user registration form and user login form?
Modifying the default user registration page
Customize user login block

Answer 1

You can add placeholders in hook_form_alter().

/**
 * Implements hook_form_alter().
 * @param $form
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 * @param $form_id
 */
function ThemeName_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'user_login_form' ) {
    // Add placeholders.
    $form['name']['#attributes']['placeholder'] = t('User name');
    $form['pass']['#attributes']['placeholder'] = t('Password');
  }
}

Use the Devel module functions to print out the form using kint, this way you can visibly see the entire structure of the $form array.

Answer 2

SimpleLogin

https://www.drupal.org/project/simplelogin

It is a simple module for Customize Drupal Login, Password and Register pages with Background images.

Administrators can provide the ability to allow users to attach their own background images/ own background color to user login, password, registration pages. Better features including customize background color, image settings.

Features: Clean & Sleek Design, Customize background color, link color, submit button color, Customize image settings, Background image Opacity, Remove unwanted Css files from simplelogin pages, Login pages wrapper width, Mobile responsive,