Drupal - How to customize the user login form?

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.

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,