How to turn a link into a button?

Question

I have a Drupal 8 site with the Private Message module. I want to display a contact button instead of the contact link.

I created a custom module, it works. But I prefer to put the code in the theme rather than creating a new module.

<?php

/**
 * @file
 * Hook implementations of private_message_contact_button module.
 */

use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
use Drupal\Core\Entity\EntityInterface;

function private_message_contact_button_node_view_alter(&$build) {
  if (array_key_exists('private_message_link', $build)) {
    $build['private_message_link']['#attributes']['class'] = array('btn-success', 'btn-sm', 'btn');
    $build['private_message_link']['#title'] = t('<i class="fas fa-comment-dots fa-lg"></i> Send a message');
  }
}

function private_message_contact_button_user_view_alter(&$build) {
  if (array_key_exists('private_message_link', $build)) {
    $build['private_message_link']['#attributes']['class'] = array('btn-success', 'btn-sm', 'btn');
    $build['private_message_link']['#title'] = t('<i class="fas fa-comment-dots fa-lg"></i> Send a message');
  }
}

I uninstalled my custom module and copied the code below in my theme but it does not work.

How to use the code in the theme ?

bootstrap_subtheme_front_office_old.theme :

/**
 * Private Message button.
 */
function bootstrap_subtheme_front_office_old_node_view_alter(&$build) {
  if (array_key_exists('private_message_link', $build)) {
    $build['private_message_link']['#attributes']['class'] = array('btn-success', 'btn-sm', 'btn');
    $build['private_message_link']['#title'] = t('<i class="fas fa-comment-dots fa-lg"></i> Send a message');
  }
}
function bootstrap_subtheme_front_office_old_user_view_alter(&$build) {
  if (array_key_exists('private_message_link', $build)) {
    $build['private_message_link']['#attributes']['class'] = array('btn-success', 'btn-sm', 'btn');
    $build['private_message_link']['#title'] = t('<i class="fas fa-comment-dots fa-lg"></i> Send a message');
  }
}