index.css

*, *::after, *::before{
    box-sizing: border-box;
    margin: 0px;
    padding:0px;
}
.light{
    --bg-color: #EFEEEE;
    --br-color: #D1CDC7;
    --tl-color: #ffffff;
    --scr-color: black;
    --scr-shadow: rgba(0,0,0,0.1);
    --switch:2;
    --switch-color: lightgreen;
}

.dark{
    --bg-color: #121212;
    --br-color: #070707;
    --tl-color: #1d1d1d;
    --scr-color: rgba(200,200,200,0.7);
    --scr-shadow: rgba(255,255,255,0.08);
    --switch:3;
    --switch-color: orange;
}

.container{
    width: 100%;
    height: 100vh;
    display: grid;
    place-content: center;
    background-color: var(--bg-color);
    font-family: monospace;
    font-size: 1.5rem;
}

.calc{
    display: grid;
    padding: 10px;
    border-radius: 5px;
    grid-template-areas: 
        "screen screen screen"
        "digits digits operator"
        "digits digits operator";
    grid-gap: 25px;
    box-shadow:  6px 6px 6px var(--br-color), 
                -6px -6px 6px var(--tl-color);
}

.digits{
    grid-area: digits;
    display: grid;
    grid-template-columns: repeat(3, minmax(50px,1fr));
    grid-auto-rows: 50px;
    grid-gap: 15px;
}

.screen{
    grid-area: screen;
    font-size: 30px;
    min-height: 60px;
    padding: 10px 15px;
    border-radius: 5px;
    color: var(--scr-color);
    text-align: right;
    text-shadow: 2px 3px var(--scr-shadow);
    box-shadow:  inset 6px 6px 5px var(--br-color), 
                inset -6px -6px 5px var(--tl-color);
}

.operator{
    grid-area: operator;
    display: grid;
    grid-template-columns: repeat(1, minmax(50px,1fr));
    grid-auto-rows: 50px;
    grid-gap: 15px;
}

.btn{
    display: grid;
    place-content: center;
    border-radius: 10px;
    box-shadow:  6px 6px 6px var(--br-color), 
                -6px -6px 6px var(--tl-color);
    transition: transform ease-in-out 100ms;
}

.btn:hover{
    cursor: pointer;
    transform: scale(1.02);
    box-shadow:  6px 6px 8px var(--br-color), 
                -6px -6px 8px var(--tl-color);
}

.btn-digi{
    color: green
}

.btn-digi-horizontal{
    grid-column: span 1
}

.btn-op{
    color: crimson;
}

.credit{
    position: absolute;
    bottom: 0;
    right:0;
    margin: 10px;
    color: grey;
    font-size: 0.8em;
    letter-spacing: 0.1rem;
}

.switch-container{
    border: 0px grey solid;
    border-radius: 25px;
    width: 70px;
    position: absolute;
    top:30px;
    right:30px;
    padding: 8px 0px;
    margin:0px;
    cursor: pointer;
    display: grid;
    place-content: center;
    grid-template-columns: 10px 1fr 1fr 10px;
    grid-template-rows: 1fr;
    background-color: var(--bg-color);
    box-shadow: inset 3px 3px 6px var(--br-color), 
                inset 3px -3px 6px var(--tl-color);
}

.switch-container:before{
    content: 'Light';
    font-size: 0.5rem;
    position: absolute;
    color: grey;
    top:-15px;
    left:0;
}

.switch-container:after{
    content: 'Dark';
    font-size: 0.5rem;
    color: grey;
    position: absolute;
    top:-15px;
    right:0;
}

.switch{
    grid-column: var(--switch);
    width: 25px;
    height: 25px;
    border-radius: 25px;
    background-color: var(--switch-color);
    box-shadow: 3px 3px 3px var(--br-color), 
                -3px -3px 3px var(--tl-color);
}