Enqueueing styles with version variable

Try this

// global theme version
$version = '1.0.0';

function frontend_styles() {

   global $version;
    wp_enqueue_style(
        'frontend_styles', //reference
        get_stylesheet_directory_uri() . '/assets/css/app.min.css', //source
        array(), //dependenices
        $version, //version number
        'all' //media type ('all', 'screen', 'handheld', 'print')
    );
}