*{
margin:0;
padding:0;
box-sizing:border-box;
font-family:'Poppins',sans-serif;
}

body{
display:flex;
justify-content:center;
align-items:center;
min-height:100vh;
background:#0f172a;
}

.calculator{
width:350px;
background:#1e293b;
padding:25px;
border-radius:20px;
box-shadow:0 0 25px rgba(0,170,255,.35);
}

#display{
width:100%;
height:70px;
border:none;
outline:none;
background:#0f172a;
color:#fff;
font-size:30px;
text-align:right;
padding:15px;
border-radius:12px;
margin-bottom:20px;
}

.buttons{
display:grid;
grid-template-columns:repeat(4,1fr);
gap:12px;
}

button{
height:65px;
border:none;
border-radius:12px;
font-size:22px;
cursor:pointer;
background:#334155;
color:white;
transition:.3s;
}

button:hover{
background:#38bdf8;
transform:scale(1.05);
}

.operator{
background:#0284c7;
}

.operator:hover{
background:#0ea5e9;
}

.equal{
background:#22c55e;
}

.equal:hover{
background:#16a34a;
}

.zero{
grid-column:span 2;
}

.back-btn{
display:block;
margin-top:20px;
text-align:center;
padding:12px;
text-decoration:none;
background:#38bdf8;
color:#fff;
border-radius:10px;
font-weight:600;
transition:.3s;
}

.back-btn:hover{
background:#0ea5e9;
}

@media(max-width:400px){

.calculator{
width:95%;
padding:20px;
}

button{
height:60px;
font-size:20px;
}

#display{
font-size:26px;
}

}