Import bootstrap 5 and bootstrap icons in wp-admin backend

Basically, to add admin-style.css to admin:

function wpdocs_enqueue_custom_admin_style() {
        wp_register_style( 'custom_wp_admin_css', get_template_directory_uri() . '/admin-style.css', false, '1.0.0' );
        wp_enqueue_style( 'custom_wp_admin_css' );
}
add_action( 'admin_enqueue_scripts', 'wpdocs_enqueue_custom_admin_style' );

Do approximately the same for Javascript. See https://developer.wordpress.org/reference/hooks/admin_enqueue_scripts/

And see https://wordpress.stackexchange.com/search?q=custom+css+admin