style.css

@charset "UTF-8";
@import url('https://fonts.googleapis.com/css2?family=Fira+Mono:wght@500&display=swap');

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    background-color: #2a3537;
    font-family: "Fira Mono", monospace, sans-serif;
    color: #fff;
    font-size: 20px;
}
.all_clear,
.plus_or_minus,
.percent,
.operator {
    color: rgb(0, 225, 255);
}
body {
    width: 100vw;
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
}
main {
    width: 280px;
    padding: 1.5rem;
    border-radius: 30px;
    box-shadow: 14px 14px 28px #14191a, -14px -14px 28px #405154;
}
input {
    outline: none;
    border: none;
    width: 100%;
    height: 30%;
    padding: 15px 30px;
    text-align: right;
    border-radius: 30px;
    box-shadow: inset 18px 18px 36px #111516, inset -18px -18px 36px #435558;
    user-select: none;
}
div {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    grid-gap: 0.8rem;
    margin-top: 30px;
}
p {
    margin: 0 auto;
    width: 40px;
    height: 40px;
    text-align: center;
    line-height: 40px;
    border-radius: 50%;
    background-color: linear-gradient(145deg, #2d393b, #263032);
    box-shadow: 5px 5px 7px #111516, -5px -5px 7px #435558;
    cursor: pointer;
    user-select: none;
}
.zero {
    grid-column: 1 / 3;
    border-radius: 30px;
    width: auto;
    margin: 0;
}
p:hover,
p:active {
    background-color: #324042;
}
p:active,
.selected {
    box-shadow: inset 22px 22px 44px #111516, inset -22px -22px 44px #435558;
}
@media (min-width: 768px) {
    body {
        font-size: 40px;
    }
    main {
        width: 430px;
        padding: 2.5rem;
    }
    div {
        margin-top: 50px;
    }
    p {
        width: 60px;
        height: 60px;
        line-height: 60px;
    }
}