修复博客主题PHP8兼容和优化深色模式

if ( empty($post_id) ) {
    $post_id = get_the_ID();
}

<span class="description"><?php echo $option['before'] ?? ''; ?></span>

echo ' <span class="desc">' . ($value['desc'] ?? '') . '</span></td></tr>';

@media (prefers-color-scheme:dark) {
    .page-indicator small {
        background: #333;
    }

    .tip-popover {
        background: var(--bg-color);
        color: #fff;
    }
}