:root{
  --bg:#0b1020;
  --panel:#11183a;
  --text:#e9eefc;
  --muted:#a8b3d6;
  --brand:#7c5cff;
  --brand2:#19d3ff;
  --danger:#ff4d4d;
  --ok:#00c853;
  --line:#25305a;
}

*{box-sizing:border-box}
body{
  margin:0;
  font-family: system-ui, -apple-system, Segoe UI, Roboto, Arial;
  background: radial-gradient(900px 400px at 20% 10%, rgba(124,92,255,.25), transparent 60%),
              radial-gradient(700px 350px at 80% 0%, rgba(25,211,255,.18), transparent 60%),
              var(--bg);
  color:var(--text);
}

.wrap{
  min-height:100vh;
  display:grid;
  place-items:center;
  padding:20px;
}

.calc{
  width:min(360px, 92vw);
  background: color-mix(in srgb, var(--panel) 90%, transparent);
  border:1px solid var(--line);
  border-radius:18px;
  padding:18px;
  box-shadow: 0 24px 60px rgba(0,0,0,.35);
}

.top{
  display:flex;
  align-items:flex-start;
  justify-content:space-between;
  gap:10px;
  margin-bottom:12px;
}

.top h1{
  font-size:16px;
  margin:0;
  color:var(--muted);
  font-weight:700;
}

.back{
  color:var(--muted);
  text-decoration:none;
  font-weight:700;
}
.back:hover{ text-decoration:underline; }

.display{
  width:100%;
  height:62px;
  border-radius:14px;
  border:1px solid var(--line);
  background: rgba(0,0,0,.18);
  color:var(--text);
  font-size:26px;
  padding:10px 12px;
  text-align:right;
  outline:none;
}

.grid{
  display:grid;
  grid-template-columns: repeat(4, 1fr);
  gap:10px;
  margin-top:14px;
}

.btn{
  height:52px;
  border:none;
  border-radius:14px;
  background: color-mix(in srgb, var(--brand) 70%, #000 30%);
  color:white;
  font-size:18px;
  font-weight:800;
  cursor:pointer;
  transition: transform .12s ease;
}
.btn:hover{ transform: translateY(-1px); }

.operator{
  background: color-mix(in srgb, var(--brand2) 70%, #000 30%);
  color:#001018;
}

.danger{ background: color-mix(in srgb, var(--danger) 85%, #000 15%); }
.equal{ background: color-mix(in srgb, var(--ok) 80%, #000 20%); }

.wide{ grid-column: span 2; }

.hint{
  margin:12px 2px 0;
  color:var(--muted);
  font-size:12.5px;
}