:root {
  --bg-start: #f3f9ff;
  --bg-end: #b7c9e8;
  --panel-start: #ffffff;
  --panel-end: #dce8ff;
  --display-bg: linear-gradient(145deg, #ffffff, #dce8ff);
  --text: #141821;
  --muted: #4f5e7f;
  --button-bg: linear-gradient(145deg, #ffffff, #dfe8f8);
  --button-text: #1b253b;
  --operator-bg: linear-gradient(145deg, #d1e2ff, #9fbbe8);
  --operator-text: #184a9b;
  --action-bg: linear-gradient(145deg, #ffdfe3, #eeadb8);
  --action-text: #a32222;
  --equals-bg: linear-gradient(145deg, #5a9dff, #2f4ed8);
  --equals-text: #ffffff;
  --shadow: 0 18px 45px rgba(25, 45, 98, 0.2);
  --ring: rgba(63, 82, 255, 0.35);
}

body[data-theme="dark"] {
  --bg-start: #0a1224;
  --bg-end: #2b3f69;
  --panel-start: rgba(28, 43, 75, 0.92);
  --panel-end: rgba(8, 17, 36, 0.94);
  --display-bg: linear-gradient(145deg, #0d1d48, #020a20);
  --text: #f4f7ff;
  --muted: #c4d3f4;
  --button-bg: linear-gradient(145deg, #3f5d98, #1f315d);
  --button-text: #edf2ff;
  --operator-bg: linear-gradient(145deg, #3f6ab3, #243f78);
  --operator-text: #dbe8ff;
  --action-bg: linear-gradient(145deg, #8a4e65, #5e3343);
  --action-text: #ffe8ee;
  --equals-bg: linear-gradient(145deg, #6dc4ff, #3c61ee);
  --equals-text: #f5fbff;
  --shadow: 0 22px 55px rgba(1, 5, 15, 0.6);
  --ring: rgba(133, 184, 255, 0.45);
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  display: grid;
  place-items: center;
  padding: 1rem;
  font-family: "Avenir Next", "Segoe UI", sans-serif;
  background: linear-gradient(140deg, var(--bg-start), var(--bg-end));
  color: var(--text);
}

.calculator {
  width: min(100%, 420px);
  background: linear-gradient(160deg, var(--panel-start), var(--panel-end));
  border: 1px solid rgba(255, 255, 255, 0.34);
  border-radius: 20px;
  box-shadow: var(--shadow);
  backdrop-filter: blur(8px);
  position: relative;
  overflow: hidden;
  padding: 1.2rem;
}

.calculator::before {
  content: "";
  position: absolute;
  inset: 0;
  pointer-events: none;
  background: linear-gradient(
    165deg,
    rgba(255, 255, 255, 0.24) 0%,
    rgba(255, 255, 255, 0.02) 45%,
    rgba(0, 0, 0, 0.08) 100%
  );
}

.topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 0.75rem;
}

h1 {
  margin: 0;
  font-size: 1.1rem;
  font-weight: 700;
  color: var(--muted);
}

.theme-toggle {
  width: 2.4rem;
  height: 2.4rem;
  border: none;
  border-radius: 999px;
  font-size: 1.05rem;
  line-height: 1;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  background: var(--button-bg);
  color: var(--button-text);
  cursor: pointer;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
}

.theme-toggle:hover {
  transform: translateY(-1px);
}

.theme-toggle:focus-visible {
  outline: 2px solid var(--ring);
  outline-offset: 2px;
}

.icon-sun,
.icon-moon {
  position: absolute;
  opacity: 0;
}

body[data-theme="light"] .icon-sun,
body:not([data-theme]) .icon-sun,
body[data-theme="dark"] .icon-moon {
  position: static;
  opacity: 1;
}

.display {
  width: 100%;
  margin-top: 0.95rem;
  min-height: 4.3rem;
  border-radius: 12px;
  padding: 0.8rem;
  background: var(--display-bg);
  font-size: clamp(1.7rem, 4.8vw, 2.4rem);
  font-weight: 600;
  text-align: right;
  overflow-x: auto;
}

.keys {
  margin-top: 1rem;
  display: grid;
  grid-template-columns: repeat(4, minmax(0, 1fr));
  gap: 0.55rem;
}

button {
  border: none;
  border-radius: 12px;
  height: clamp(2.9rem, 10vw, 3.4rem);
  font-size: clamp(1rem, 3.8vw, 1.15rem);
  font-weight: 700;
  cursor: pointer;
  background: var(--button-bg);
  color: var(--button-text);
  transition: transform 0.14s ease, filter 0.14s ease;
  box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.28);
}

button:hover {
  filter: brightness(1.05);
}

button:active {
  transform: translateY(1px);
}

button:focus-visible {
  outline: 2px solid var(--ring);
  outline-offset: 2px;
}

.operator {
  background: var(--operator-bg);
  color: var(--operator-text);
}

.action {
  background: var(--action-bg);
  color: var(--action-text);
}

.equals {
  grid-row: span 2;
  height: auto;
  background: var(--equals-bg);
  color: var(--equals-text);
}

.zero {
  grid-column: span 2;
}

@media (max-width: 480px) {
  body {
    padding: 0.75rem;
  }

  .calculator {
    border-radius: 16px;
    padding: 0.95rem;
  }

  .keys {
    gap: 0.45rem;
  }
}