Add dynamic version number to WordPress style.css

functions.php

<?php
function your_theme_styles() {
    wp_enqueue_style( 
        'style', 
        get_stylesheet_directory_uri() . '/style.css', 
        false, 
        date( 'dmYhi', filemtime( get_stylesheet_directory() . '/style.css' ) ) 
    );
}

add_action( 'wp_enqueue_scripts', 'your_theme_styles' );