How to include custom CSS in the Back Office of PrestaShop

prestashop_back_office_css.php

<?php
public function install()
{    
    if ($this->registerHook('actionAdminControllerSetMedia')) {
        return true;
    }

    $this->uninstall();

    return false;
}

public function hookActionAdminControllerSetMedia() {

    // Add the path to your CSS file
    // path_to_modules/this_module_name/path_to_css_file.js
    $this->context->controller->addCSS(__PS_BASE_URI__ . 'modules/' . $this->name . '/' . 'file_name.css');
}