/*新文章发布角标*/
function add_zbfox_new_post_label($title, $id) {
    if (!is_admin() && !is_single()) {
        $post_time = get_the_time('U', $id);
        if (time() - $post_time <= 24 * 60 * 60) {
            $title .= ' <div class="zbfox_new_post_label">NEW</div>';
        }
    }
    return $title;
}
add_filter('the_title', 'add_zbfox_new_post_label', 10, 2);

最新版CSS代码添加到子比主题后台设置->自定义代码->自定义CSS样式

/*新文章发布图标样式*/
.posts-item{
    position: relative;
    overflow: visible; 
}

.zbfox_new_post_label{
    position: absolute;
    top: 15px;
    right: 0; 
    padding: 5px 10px;
    background: #4f15ff;
    box-shadow: -1px 2px 4px rgba(0, 0, 0, 0.5);
    /*url: huliku.com*/
    color: #fff;
    font-size: 13px;
    font-weight: 900;
    border-radius: 5px 0 0 5px;
}

.posts-item:hover .zbfox_new_post_label {
    opacity: 0;
    transition: opacity 0.5s ease;
}

.zbfox_new_post_label:before {
    position: absolute;
    content: " ";
    display: block;
    width: 7px;
    height: 110%;
    padding: 0 0 7px;
    top: 0;
    right: -7px; 
    background: inherit;
    border-radius: 0 5px 5px 0;
}

.zbfox_new_post_label:after {
    position: absolute;
    content: " ";
    display: block;
    width: 5px;
    height: 5px;
    background: rgba(0, 0, 0, 0.35);
    bottom: -3px; 
    right: -5px; 
    border-radius: 0 5px 5px 0;
}