style.css

body {
    --bg-color: var(--tg-theme-bg-color);
    font: 12px/18px "Lucida Grande", "Lucida Sans Unicode", Arial, Helvetica, Verdana, sans-serif;
    background-color: var(--bg-color);
    color: var(--tg-theme-text-color);
    margin: 48px 24px;
    padding: 0;
    color-scheme: var(--tg-color-scheme);
}

a {
    color: var(--tg-theme-link-color);
}
#viewport {
    position: fixed;
    left: 0;
    right: 0;
    top: 0;
    height: var(--tg-viewport-stable-height, 100vh);
    pointer-events: none;
    transition: all .2s ease;
}
#viewport:after {
    content: '';
    display: block;
    position: absolute;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
    border-width: 4px;
    border-style: solid;
    border-image: linear-gradient(45deg, rgba(64, 224, 208, .5), rgba(173, 255, 47, .5)) 1;
}

#viewport-params-size,
#viewport-params-expand {
    content: attr(text);
    position: absolute;
    display: inline-block;
    background: var(--tg-theme-link-color, rgb(64, 224, 208));
    right: 4px;
    left: auto;
    font-size: 8px;
    padding: 4px;
    vertical-align: top;
}
#viewport-params-size {
    top: 4px;
}
#viewport-params-expand {
    top: 30px;
}