Derive version number from WordPress theme style.css

derive-version-number.php

<?php

/**
 * Get theme version number from WP_Theme object (cached)
 */
function gpp_get_theme_version() {
    $theme_file = get_template_directory() . '/style.css';
    $theme = new WP_Theme( basename( dirname( $theme_file ) ), dirname( dirname( $theme_file ) ) );

    return $theme->get( 'Version' );
}