style.css

body {
    position: fixed;
    top: 0;
    left: 0;
    padding: 0;
    margin: 0;
    width: 100%;
    height: 100%;
    background-color: #0d1117;
    display: flex;
    flex-direction: column;
}

body > header {
    width: calc(100% - 2*16px);
    padding: 32px 16px;
    background-color: #161b22;
}
body > header > a {
    font-family: sans-serif;
    font-size: 24px;
    font-weight: bold;
    text-decoration: none;
    color: #f0f6fc;
    user-select: none;
}

body > main {
    display: flex;
    flex-direction: column;
}
body > main > #input {
    border: 0;
    outline: none;

    margin: 16px 8px;
    border: 1px solid #30363d;
    border-radius: 8px;
    padding: 4px;
    width: calc(100% - 2*8px - 2*1px - 2*4px);
    height: 330px;

    font-family: sans-serif;
    font-size: 18px;
    font-weight: normal;
    color: #f0f6fc;

    background-color: #0d1117;

    resize: none;
}
body > main > #output {
    border: 0;
    outline: none;

    margin: 0;
    margin-left: 7.5%;
    border-radius: 8px;
    padding: 6px;
    width: calc(85% - 2*6px);

    font-family: sans-serif;
    font-size: 18px;
    font-weight: normal;
    color: #f0f6fc;
    text-overflow: ellipsis;

    background-color: #30363d;
    transition: background-color 0.3s linear;

    cursor: pointer;
    resize: none;
}