只需在li class属性中添加条件即可，我个人会在设置$_POST['doneButton'])的条件下使用如下三元运算符添加MYNEWCLASS：

<li class="note<?=(isset($_POST['doneButton'])?" MYNEWCLASS":"");?>" id='<?= $item['id'] ?>'>

编辑：通过特定ID选择LI：

<?php
<ul class='wrapper'> 
<?php foreach ($items as $item) : ?>

    <?php
        $class = ""; //Declare an empty $class
        if (isset($_POST['doneButton']) && $_POST['doneButton'] == $item['id']){
            // If $_POST['doneButton'] is set, and $_POST['doneButton'] is equal to the id of the current iteration then set $class to "MYNEWCLASS"
            $class = " MYNEWCLASS"; //THE SPACE IS IMPORTANT
        }
    ?>

    <li class="note<?=$class;?>" id='<?= $item['id'] ?>'>
        <div class="details">
            <p><?= $item['title']."<span class='currentDate'>".$item['currentDate']."</span>"?></p>
            <p class ="importance"><?= $item['importance'] ?></p>
            <div class="border"></div>
            <span><?= $item['text'] ?></span>
        </div>
        <div class="bottom-content">
            <span>Do: <?= date('d.m.Y', strtotime($item['date'])) ?></span>

            <form method="post">
            <button name="doneButton" value='<?= $item['id'] ?>' type="submit">✅</button>
            </form>

            <form method="post" >
            <button value="<?= $item['id'] ?>" name="deleteButton" type="submit">🗑️</button>
            </form>
            <?php
                if(isset($_POST['deleteButton'])){
                    db_remove_note($_POST['deleteButton']);
                    header("Location: list-notes.php");
                }
            ?>
        </div>
    </li>
<?php
endforeach;
?>
</ul>