style.css

body{
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: antiquewhite;
}
.container{
    background-color: hsl(0, 0%, 15%);
    border-radius: 15px;
    max-width: 500px;
    overflow: hidden;
}
#data{
    width: 100%;
    padding: 25px;
    font-size: 3em;
    border: none;
    text-align: center;
    background-color: hsla(0, 0%, 20%);
    color: whitesmoke;
}
#keys{
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    padding: 30px;
}
button{
    width: 100px;
    height: 100px;
    border-radius: 45%;
    border: none;
    background-color: hsla(0, 0%, 40%);
    color: white;
    font-size: 3.5rem;
    cursor: pointer;
}
button:hover{
    background-color: hsla(0, 0%, 50%);    
}
button:active{
    background-color: hsla(0, 0%, 65%);    
}
.operatorbtn{
    background-color: hsl(39, 100%, 53%);
}
.operatorbtn:hover{
    background-color: hsl(39, 100%, 63%);
}
.operatorbtn:active{
    background-color: hsl(39, 100%, 78%);
}