style.css

* {
    box-sizing: border-box;
}

html {
    background-color: #F6F6F6;
    font-size: 16px;
    margin-left: calc(100vw - 100%);
    margin-right: 0;
}

body {
    color: rgb(73, 78, 82);
    font-family: Carlito, Calibri, sans-serif;
    line-height: 1.5;
}

.msg {
    width: 70ch;
    border-radius: 5px;
    padding: 0.8em;
    background-color: #cecace;
}

.msg.me {
    margin-left: 300px;
    background-color: #bfd9d7;
}

h1 {
    font-size: 2em;
}

a {
    color: rgb(52, 105, 160);
    text-decoration: none;
}

a:hover {
    text-decoration: underline;
}

div {
    margin-bottom: 2em;
}

.date, .time {
    float: right;
}

.time {
    clear: right;
}

.sender {
    font-weight: bold;
}

.body {
    display: block;
    margin-left: 3em;
    word-wrap: break-word;
}

figure > label > img, video {
    display: block;
    width: 300px;
}

label > img {
    cursor: pointer;
}

.modal {
    opacity: 0;
    visibility: hidden;
    position: fixed;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    text-align: left;
    background: rgba(0, 0, 0, 0.8);
    transition: opacity .25s ease;
    z-index: 99;
}

.modal-content {
    width: 100vw;
    height: 100vh;
}

.modal-photo {
    width: 70%;
    max-height: 80%;
    position: absolute;
    left: 50%;
    top: 10%;
    margin-left: -35%;
    object-fit: scale-down;
}

.modal-state {
    display: none;
}

.modal-state:checked + .modal {
    opacity: 1;
    visibility: visible;
}