style.css

*{
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}
body{ 
    display: flex;
    justify-content: center;
    align-items: center;
    width: 100vw;
    height: 100vh;
    overflow: hidden;
    background-color: #d6d6d6;
    font-family: monospace;
}
.calculator{
    width: 300px;
    padding: 20px;
    border-radius: 15px;
    background-color: #C0C0C0;
    box-shadow: 5px 5px 10px #b6a9a9, -5px -5px 10px #ffffff;
}
.display{
    width: 100%;
    border: none;
    outline: none;
    padding: 20px;
    margin: 20px 0 20px 0;
    text-align: right;
    font-size: 20px;
    font-weight: bold;
    border-radius: 10px;
    box-shadow: inset 2px 2px 5px #babecc, inset -5px -5px 10px #ffffff;
}
.keys{
    margin: 20px 0 20px 0;
}
.row{
    display: flex;
    justify-content: space-between;
}
.num{
    height: 45px;
    width: 50px;
    border: none;
    outline: none;
    margin: 10px 0 5px 5px;
    font-size: 20px;
    font-weight: bold;
    border-radius: 20%;
    background: rgb(200, 200, 200);
    box-shadow: 5px 5px 8px #b6acac, -5px -5px 8px #faf4f4;
}
button:hover {
    cursor: pointer;
}
button:active {
    box-shadow: inset 1px 1px 2px #babeec, inset -1px -1px 2px #ffffff;
}
.operator{
    background: grey;
    color: #ffffff;
}
#result{
    width: 120px;
    border-radius: 20px;
}