WordPress adds ?ver=X.X to CSS and JS files. Removing the version parameter can improve caching. Add to your functions.php file. For more tips on removing scripts and styles.
add_filter( 'script_loader_src', function( $src ) {
return remove_query_arg( 'ver', $src );
}, 15, 1 );
add_filter( 'style_loader_src', function( $src ) {
return remove_query_arg( 'ver', $src );
}, 15, 1 );