style.css

/* resets */

*,
*::before,
*::after {
    box-sizing: border-box;
}

* {
    margin: 0;
}

body {
    line-height: 1.5;
    -webkit-font-smoothing: antialiased;
}

input,
button,
textarea,
select {
    font: inherit;
}

p,
h1,
h2,
h3,
h4,
h5,
h6 {
    overflow-wrap: break-word;
}

/* main styles */

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    font-family: 'Franklin Gothic Medium', 'Arial Narrow', Arial, sans-serif;
    background-color: #555;
}

.container {
    width: 464px;
    height: 464px;
    padding: 16px;
    border-radius: 16px;
    color: #9C27B0;
    background-color: #E1BEE7;
    display: flex;
    flex-direction: column;
}

.container__display {
    font-size: 48px;
    margin: 16px 0;
    background-color: #fff;
    border-radius: 8px;
    flex: 1;
    display: flex;
    align-items: center;
    justify-content: flex-end;
    padding: 0 12px;
    overflow: hidden;
}

.container__buttons {
    display: flex;
    flex-direction: column;
    gap: 4px;
    flex: 4;
}

.container__buttons__group {
    display: flex;
    gap: 4px;
    flex: 1;
}

.container__buttons__group button {
    flex: 1;
    border-radius: 8px;
    font-size: 32px;
    border: none;
    color: #9C27B0;
    background-color: #fff;
}

.container__buttons__group button:hover {
    background-color: #eee;
}

#clear {
    color: #de1738;
}

#equals {
    background-color: #9C27B0;
    color: #fff;
}

button.operator {
    color: #555;
}