<div class="menu">
        <li class="item" id="mn1">
            <a class="btn" href="#mn1">Kullanıcılar</a>
            <div class="submenu">
                <a href="#">Kullanıcı Listesi</a>
                <a href="#">Kullanıcı Ekle</a>
                <a href="#">Kullanıcı Sil</a>
            </div>
        </li>
        <li class="item" id="mn2">
            <a class="btn" href="#mn2">Dosyalar</a>
            <div class="submenu">
                <a href="#">Dosya Listesi</a>
                <a href="#">Dosya Ekle</a>
                <a href="#">Dosya Sil</a>
            </div>
        </li>
        <li class="item" id="mn3">
            <a class="btn" href="#mn3">Ayarlar</a>
            <div class="submenu">
                <a href="#">Kullanıcı Ayarları</a>
                <a href="#">Dosya Ayarları</a>
                <a href="#">Şifre</a>
            </div>
        </li>
    </div>

:root{
    font-family: Arial, Helvetica, sans-serif;
    font-size: 15px;
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

*{
    text-decoration: none;
    color:white;
    list-style: none;
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body{
    display:flex;
    align-items: center;
    justify-content: center;
    min-height: 100vh;
    background: #555;
    overflow: hidden;
}

.menu{
    width: 300px;
    height: auto;
    background: #162447;
    border-radius: 8px;
    overflow: hidden;
    box-shadow: 0 0 10px rgba(0, 0, 0, .5);
}

.menu .btn{
    display:block;
    padding: 1rem;
    border-bottom: solid 1px #1b1b2f;
    border-top: solid 1px #1f4068;
    position: relative;
}

.menu .submenu {
    background: #1b1b2f;
    overflow: hidden;
    max-height: 0;
    transition: max-height .8s ease-out;
}

.menu .submenu a{
    display:block;
    padding: 1rem;
    position: relative;
}

.menu .submenu a::before{
    content:'';
    display:block;
    position: absolute;
    top:0;
    left:0;
    height: 100%;
    width: 5px;
    background: #e43f5a;
    opacity: 0;
    transition: all .5s;
}

.menu .submenu a:hover{
    padding-left: calc(1rem + 5px);
}

.menu .submenu a:hover::before{
    opacity: 1;
}

.item:target .submenu{
    max-height: 20rem;
}