* {
  box-sizing: border-box;
  font-family: -apple-system, BlinkMacSystemFont, sans-serif;
}

:root {
  --bg: #000;
  --display: #fff;
  --btn-dark: #333;
  --btn-gray: #a5a5a5;
  --btn-orange: #ff9f0a;
  --history-bg: #111;
}

.light {
  --bg: #f2f2f7;
  --display: #000;
  --btn-dark: #fff;
  --btn-gray: #d1d1d6;
  --btn-orange: #ff9f0a;
  --history-bg: #e5e5ea;
}

body {
  background: var(--bg);
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  transition: background 0.3s ease;
}

.calculator {
  width: 320px;
  padding: 20px;
  background: var(--bg);
  border-radius: 30px;
  position: relative;
  transition: background 0.3s ease;
}

.expression {
  min-height: 22px;
  margin-bottom: 4px;
  text-align: right;
  font-size: 18px;
  color: var(--display);
  opacity: 0.6;
  transition: opacity 0.2s ease;
}

.display {
  color: var(--display);
  font-size: 64px;
  text-align: right;
  padding: 20px 10px;
}

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

.btn {
  height: 65px;
  border-radius: 50%;
  border: none;
  font-size: 26px;
  cursor: pointer;
  transition: transform 0.1s ease, filter 0.1s ease;
}

.btn:active {
  transform: scale(0.93);
  filter: brightness(1.2);
}

.dark {
  background: var(--btn-dark);
  color: var(--display);
}

.gray {
  background: var(--btn-gray);
  color: #000;
}

.orange {
  background: var(--btn-orange);
  color: #fff;
}

.zero {
  grid-column: span 2;
  border-radius: 40px;
  text-align: left;
  padding-left: 25px;
}

.history-container {
  position: absolute;
  top: 10px;
  left: 15px;
}

#historyBtn {
  background: none;
  border: none;
  font-size: 22px;
  cursor: pointer;
  color: var(--display);
}

.history {
  position: absolute;
  top: 30px;
  left: 0;
  width: 260px;
  max-height: 220px;
  padding: 10px;
  border-radius: 10px;
  background: var(--history-bg);
  overflow-y: auto;
  animation: slideDown 0.25s ease;
}

.history-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 8px;
  padding-bottom: 6px;
  border-bottom: 1px solid rgba(255,255,255,0.1);
}

.history-header span {
  font-size: 14px;
  color: var(--display);
  opacity: 0.8;
}

#clearHistory {
  background: none;
  border: none;
  font-size: 16px;
  cursor: pointer;
  color: var(--display);
  opacity: 0.7;
  transition: opacity 0.2s ease, transform 0.2s ease;
}

#clearHistory:hover {
  opacity: 1;
}

#clearHistory:active {
  transform: scale(0.9);
}

.history div {
  color: #888;
  font-size: 14px;
  margin-bottom: 6px;
}

.hidden {
  display: none;
}

@keyframes slideDown {
  from {
    opacity: 0;
    transform: translateY(-10px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

.top-bar {
  display: flex;
  justify-content: flex-end;
  margin-bottom: 5px;
}

#themeToggle {
  background: none;
  border: none;
  font-size: 20px;
  cursor: pointer;
  color: var(--display);
  transition: transform 0.2s ease;
}

#themeToggle:active {
  transform: scale(0.9);
}

@media (max-width: 400px) {
  .calculator {
    width: 100%;
    height: 100%;
    border-radius: 0;
  }
}