Enqueing via last modified

enqueue-modified-time.php

<?php

function my_theme_get_version() {
  return filemtime( get_template_directory() . '/assets/styles/main.css' );
}

function my_theme_setup_styles() {
  wp_register_style(
    'main',
    get_template_directory_uri() . '/assets/styles/main.css',
    [ ],
    my_theme_get_version(),
  );

  wp_enqueue_style( 'main' );
}