:root {
    font-size: 36px;
}
* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    transition: 400ms;
}

body {
    height: 100vh;
    width: 100vw;
    display: flex;
    margin-top: 3rem;
    flex-direction: column;
    align-items: center;
    background-color: #191919;
    overflow: hidden;
}
.output {
    animation: fadeIn 2s;
    font-size: 3rem;
    color: #999999
}
.increment {
    animation: fadeIn 3s;
    margin: 20px 0;
}
button {
    height: 4rem;
    width: 6rem;
    background: transparent;
    outline: none;
    border: none;
    font-size: 3rem;
}
.clear {
    animation: fadeIn 5s;
    font-size: 2rem;
}
.add {
    color: green;
}
.add:hover {
    text-shadow: 0 0 32px yellowgreen;
    color: yellowgreen;
}
.sub {
    color: red;
}
.sub:hover {
    text-shadow: 0 0 32px tomato;
    color: tomato;
}
.clear:hover {
    color: white;
    opacity: .1;
}

@keyframes fadeIn {
    0% {
        opacity: 0%;
    }
    25% {
        opacity: 0%;
    }
    100% {
        opacity: 100%;
    }
}