How to customize the user login form? 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(&amp;$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
You can add placeholders in hook_form_alter().

/**
 * Implements hook_form_alter().
 * @param $form
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 * @param $form_id
 */
function ThemeName_form_alter(&amp;$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.