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

#calculator{
    font-family:Arial, sans-serif;
    background-color:hsl(0,0%,15%);
    border-radius:15px;
    max-width:500px;
    overflow:hidden;
    box-shadow:0 10px 25px rgba(0,0,0,0.3);
}

#display{
    width:100%;
    padding:20px;
    box-sizing:border-box;
    font-size:3rem;
    text-align:left;
    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:none;
    border-radius:50%;
    background-color:hsl(0,0%,30%);
    color:white;
    font-size:3rem;
    font-weight:bold;
    cursor:pointer;
    transition:0.2s;
}

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

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

.operator-btn{
    background-color:hsl(233,79%,45%);
}

.operator-btn:hover{
    background-color:hsl(214,65%,55%);
}

.operator-btn:active{
    background-color:hsl(228,95%,75%);
}

/* HISTORY */

#history{
    width:500px;
    margin-top:20px;
    font-family:Arial, sans-serif;
}

#history h3{
    margin:0 0 10px;
    color:black;
}

#history-list{
    list-style:none;
    padding:0;
    margin:0;
}

#history-list li{
    background:white;
    padding:10px 15px;
    margin-bottom:8px;
    border-radius:8px;
    box-shadow:0 2px 6px rgba(0,0,0,0.15);
    color:black;
}