您可以将发布日期与所选日期进行比较，并根据结果将不同的样式排队：

<?php

function my_enqueue_styles_depending_on_date() {

    global $post;

    $style_switch_date = \'20201230\'; // the trigger date
    
    if ( is_single( $post->ID ) ) {

        $post_date = get_the_date( \'Ymd\', $post->ID );

        if ( $post_date > $style_switch_date ) {
            wp_enqueue_style( \'css_for_newer_posts\' );
        } else {
            wp_enqueue_style( \'css_for_older_posts\' );
        }
    }
}

add_action(\'wp_enqueue_scripts\', \'my_enqueue_styles_depending_on_date\');