style.css

@import url('https://fonts.googleapis.com/css2?family=Roboto+Mono&display=swap');

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

html {
    font-size: 16px;
    font-family: "Roboto Mono", monospace;
    color: #000;
}

body {
    width: 100vw;
    height: 100vh;
    background: #fff;
    background: linear-gradient(
    90deg,
    rgba(255, 255, 255, 1) 0%,
    rgba(70, 130, 180, 1) 0%,
    rgba(238, 238, 238, 1) 50%,
    rgba(255, 165, 0, 1) 100%
    );
    display: grid;
    place-content: center;
}

.container {
    width: 90vw;
    max-width: 500px;
    background-color: #cecece;
    display: grid;
    gap: 1px;
    grid-template-areas: 
        "header header header header"
        "display display display display";
    grid-template-columns: repeat(4, minmax(0, 1fr));
    grid-template-rows: 20px minmax(120px, auto) repeat(5, minmax(0, 70px));
}

.container:focus-within {
    background-color: rgba(225, 225, 225, 0.5);
}

.header {
    grid-area: header;
    padding: 0.25rem 0.5rem;
    display: flex;
    align-items: center;
}

.header > h1 {
    font-family: 'Franklin Gothic Medium', 'Arial Narrow', Arial, sans-serif;
    font-size: 0.85rem;
    font-weight: normal;
}

.display {
    grid-area: display;
    padding: 3px 5px;
    display: flex;
    justify-content: space-between;
    flex-direction: column;
}

.previousvalue {
    display: flex;
    justify-content: flex-end;
    align-items: center;
    height: 1rem;
}

.currentvalue {
    font-size: 3rem;
    font-weight: bolder;
    background-color: transparent;
    border: none;
    outline: none;
    text-align: right;
    font-family: "Roboto Mono", monospace;
}

button {
    display: grid;
    place-content: center;
    font-size: 1.5rem;
    padding: 0.5rem;
    background-color: whitesmoke;
    border: none;
    outline: none;
}

button:hover, 
button:active {
    background-color: rgba(255,255,255, 0.2);
}

.function {
    background-color: rgb(235, 235, 235);
}

.equals {
    background-color: rgb(108, 177, 233);
}

.equals:hover, 
.equals:active {
    background-color: steelblue;
    filter: brightness(110%);
}

@media screen and (min-width: 768px) {
    html {
        font-size: 22px;
    }
    .container {
        grid-template-rows: 35px minmax(95px, auto) repeat(5, minmax(0, 95px));
    }
}