la-visitor/theme/style/app.css

body {
    margin: 0px;
    padding: 0px;
}

.app { 
    position: fixed;
    width: 100%;
    height: 100%;
    top: 0px;
    left: 0px;
    margin: 0px;
    padding: 0px;
    color: var(--font-color);
    background-color: var(--background-color);

    transition:
        background-color 0.5s,
        color 0.5s;
}

#push-change-color {
    position: fixed;
    right: 20px;
    bottom: 20px;
}