style.css

*{
    margin:0;
    padding: 0;
    box-sizing: border-box;
    background-color:#e9e9e9;
    font-size: 20px;
    font-family: 'sans-serif',Fira Sans ;
}

body{
    height: 100vh;
    width:100vw;
    display: flex;
    justify-content: center;
    align-items: center;  
}
main{
    padding:2rem;
    border-radius: 30px;
    box-shadow:-8px -8px 10px #fff, 4px 4px 15px rgb(0 0 0 / 40%);
}
#screen{
    width:300px;
    height: 50px;
    border:none;
    border-radius: 30px;
    text-align: right;
    box-shadow:inset -11px -11px 10px #fff, inset 4px 4px 15px rgb(0 0 0 / 40%);
    padding:15px 15px;
    text-shadow:-1px -1px 2px #fff,1px 3px 2px rgba(0,0,0,0.4);
    outline: none; //hides the outline when clicked 
}
.keys{
    display:grid;
    grid-template-columns: repeat(4,1fr);
    grid-gap:0.5rem;
    margin-top:25px;  
}
.keys button{
    cursor: pointer;
    border-radius: 30px;
    height: 60px;
    width: 60px;
    box-shadow:-6px -7px 10px #fff, 2px 2px 10px rgb(0 0 0 / 40%);
    border:none;
    background-color: #e9e9e9;
}
.keys button:hover{
    background: #ccc;
}
.keys button:active{
    box-shadow:-11px -11px 10px #fff,4px 4px 15px rgb(0 0 0 / 40%);
}
.operator,.equality{
    color:red;
    font-weight: bold;
}
.number{
    font-weight: bold;
}