body{
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    height:100vh;
    background-color: hsl(0,0%,95%);
}


#calculator{
   font-family: Arial, Helvetica, sans-serif;
   background-color: hsl(0,0%,15%);
   border-radius: 15px;
   max-width:500px;
   overflow: hidden; 
}
#display{
    width:100%;
    padding:20px;
    font-size:5rem;
    border:none;
    background-color: hsl(0,0%,20%);
    color:white;
}


#keys{
    display:grid;
    grid-template-columns: repeat(4,1fr);
    gap:10px;
    padding:25px;
}


button{
    width:100px;
    height:100px;
    border-radius:50px;
    border:none;
    background-color: hsl(0,0%,30%);
    color:white;
    font-size:3rem;
    font-weight: bold;
    cursor:pointer;
}

button:hover{
    background-color: hsl(0,0%,40%);
}

button:active{
    background-color: hsl(0,0%,50%);
}

.operator-btn{
    background-color: hsl(35,100%,55%);
}
.operator-btn:hover{
    background-color: hsl(35,100%,65%);
}
.operator-btn:active{
    background-color: hsl(35,100%,75%);
}

#back{
    position: fixed;
    top:0;
    right:0;
}
#back button{
    background-color: black;
}

#back a{
    font-size: medium;
    color:white;
    text-align: center;
}

@media (max-width: 480px) {
  #calculator {
    max-width: 300px;
    max-height:100vh;     
    border-radius: 10px;
  }

  #display {
    font-size: 3rem;     
    padding: 15px;
  }

  #keys {
    gap: 8px;
    padding: 15px;
  }

  button {
    width: 50px;      
    border-radius: 30px;
    font-size: 2rem;
  }
}