styles.css

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    gap: 1em;
    min-height: 100dvh;
    background: linear-gradient(to right, rgb(0 150 255) 16%, rgb(0 255 150 / .7))
}

.title {
    color: #fff;
    font-size: 2.5rem;
    font-family: sans-serif;
    letter-spacing: .3rem;
}

.calculator-grid {
    display: grid;
    justify-content: center;
    align-content: center;
/*     min-height: 100vh; */
    grid-template-columns: repeat(4, 80px);
    grid-template-rows: minmax(120px, auto) repeat(5, 80px);
}

.calculator-grid > .output {
    grid-column: 1 / -1;
    background: rgb(0 0 0 / .78);
    display: flex;
    word-break: break-word;
    flex-direction: column;
    justify-content: space-between;
    align-items: flex-end;
    padding: .5em 1em;
    letter-spacing: .1rem;
    position: relative;
    overflow: clip;
}

.calculator-grid > .output.animate::before {
    content: "";
    position: absolute;
    bottom: 0;
    right: 0;
    width: 40px;
    aspect-ratio: 1;
    background: #fff;
    border-radius: 50%;
    transform: translate(7px, 10px);
    opacity: 0;
    animation: enlargen 400ms ease-in-out;
}

@keyframes enlargen {
    0%{
     opacity: 1;   
    }

    75% {
        opacity: 0.8;
        transform: scale(100);
    }

    90% {
       opacity: 0.5; 
    }

    100% {
        opacity: 0;
    }
}

.output > .previous-operand {
    color: rgb(255 255 255 / .7);
    font-size: 2rem;
}

.output > .current-operand {
    color: #fff;
    font-size: 3rem;
}

.calculator-grid > button {
    border: 1px solid rgb(255 255 255);
    cursor: pointer;
    font-size: 1.5rem;
    background-color: rgb(255 255 255 / .75);
}

.calculator-grid > button:hover {
    background-color: #fff;
}

.calculator-grid > .span-two {
    grid-column: span 2;
}

.calculator-grid > button[data-equals] {
    background: #fff;
}