style.css

html,
body {
    padding: 0;
    margin: 0;
    font-family: -apple-system, BlinkMacSystemFont, Segoe UI, Roboto, Oxygen, Ubuntu, Cantarell, Fira Sans, Droid Sans, Helvetica Neue, sans-serif;
}

a {
    color: inherit;
    text-decoration: none;
}

* {
    box-sizing: border-box;
}

.column {
    float: left;
    width: 33.33%;
    padding: 8px;
}


/* Clearfix (clear floats) */

.row::after {
    content: "";
    clear: both;
    display: table;
}

.container {
    min-height: 100vh;
    padding: 0 0.5rem;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
}

.main {
    padding: 5rem 0;
    flex: 1;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    width: 100%;
}

.footer {
    width: 100%;
    height: 100px;
    border-top: 1px solid #eaeaea;
    display: flex;
    justify-content: center;
    align-items: center;
}

.footer img {
    margin-left: 0.5rem;
}

.footer a {
    display: flex;
    justify-content: center;
    align-items: center;
}

.title a {
    color: #0070f3;
    text-decoration: none;
}

.title a:hover,
.title a:focus,
.title a:active {
    text-decoration: underline;
}

.title {
    margin: 0;
    line-height: 1.15;
    font-size: 4rem;
    text-align: center;
}

.logo {
    height: 1em;
}

.visually-hidden {
    position: absolute !important;
    height: 1px;
    width: 1px;
    overflow: hidden;
    clip: rect(1px, 1px, 1px, 1px);
}

input.visually-hidden+label {
    background-color: #0070f3;
    border-radius: 5px;
    color: white;
    padding: 5px 10px;
    cursor: pointer;
}

input.visually-hidden+label:hover {
    background-color: #1373e4;
}

input.visually-hidden+label:active {
    background-color: #1f6bc3;
}

input.visually-hidden:focus+label {
    outline: thin dotted;
}

input.visually-hidden:focus-within+label {
    outline: thin dotted;
}

.operating {
    margin: 2rem 0;
}

.operating>div {
    width: 100%;
    text-align: center;
}

.operating button {
    background-color: #0070f3;
    border-radius: 5px;
    color: white;
    padding: 5px 10px;
    cursor: pointer;
    border: 0;
    font-size: 0.9em;
}

.operating button:hover {
    background-color: #1373e4;
}

.operating button:active {
    background-color: #1f6bc3;
}

.operating button:disabled {
    background-color: #eaeaea;
    cursor: not-allowed;
}

.noselect {
    -webkit-touch-callout: none;
    /* iOS Safari */
    -webkit-user-select: none;
    /* Safari */
    -khtml-user-select: none;
    /* Konqueror HTML */
    -moz-user-select: none;
    /* Old versions of Firefox */
    -ms-user-select: none;
    /* Internet Explorer/Edge */
    user-select: none;
    /* Non-prefixed version, currently
                                    supported by Chrome, Edge, Opera and Firefox */
}

.thumb {
    margin-top: 1rem;
}

.thumb img {
    max-width: 800px;
}

.infer {
    margin: 1rem 0;
}

.infer a {
    color: #1f6bc3;
    text-decoration: underline;
}