:root {
  /* Default Theme: Matcha (Soft Green/Cream) */
  --bg-color: #f0fdf4;
  --calc-bg: #ffffff;
  --calc-shadow: rgba(134, 239, 172, 0.4);
  --text-main: #166534;
  --text-muted: #86efac;
  
  --btn-num-bg: #f8fafc;
  --btn-num-text: #1e293b;
  --btn-num-shadow: #e2e8f0;
  
  --btn-op-bg: #dcfce7;
  --btn-op-text: #15803d;
  --btn-op-shadow: #bbf7d0;
  
  --btn-act-bg: #f1f5f9;
  --btn-act-text: #475569;
  --btn-act-shadow: #cbd5e1;

  --btn-eq-bg: #22c55e;
  --btn-eq-text: #ffffff;
  --btn-eq-shadow: #16a34a;

  --display-bg: #f0fdf4;
  --display-shadow: inset 0 2px 5px rgba(0,0,0,0.05);
}

[data-theme="sakura"] {
  --bg-color: #fdf2f8;
  --calc-bg: #ffffff;
  --calc-shadow: rgba(249, 168, 212, 0.4);
  --text-main: #9d174d;
  --text-muted: #f472b6;
  
  --btn-num-bg: #fff1f2;
  --btn-num-text: #4c1d95;
  --btn-num-shadow: #ffe4e6;
  
  --btn-op-bg: #fce7f3;
  --btn-op-text: #be185d;
  --btn-op-shadow: #fbcfe8;
  
  --btn-act-bg: #f8fafc;
  --btn-act-text: #64748b;
  --btn-act-shadow: #e2e8f0;

  --btn-eq-bg: #ec4899;
  --btn-eq-text: #ffffff;
  --btn-eq-shadow: #be185d;

  --display-bg: #fdf2f8;
}

[data-theme="taro"] {
  --bg-color: #1e1b4b;
  --calc-bg: #2e1065;
  --calc-shadow: rgba(0, 0, 0, 0.5);
  --text-main: #ddd6fe;
  --text-muted: #8b5cf6;
  
  --btn-num-bg: #4c1d95;
  --btn-num-text: #f5f3ff;
  --btn-num-shadow: #3b0764;
  
  --btn-op-bg: #7c3aed;
  --btn-op-text: #ffffff;
  --btn-op-shadow: #5b21b6;
  
  --btn-act-bg: #3b2c4e;
  --btn-act-text: #a78bfa;
  --btn-act-shadow: #2e1065;

  --btn-eq-bg: #c4b5fd;
  --btn-eq-text: #2e1065;
  --btn-eq-shadow: #8b5cf6;

  --display-bg: #1e1b4b;
  --display-shadow: inset 0 4px 10px rgba(0,0,0,0.3);
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  -webkit-tap-highlight-color: transparent;
  user-select: none;
}

body {
  background-color: var(--bg-color);
  min-height: 100vh;
  min-height: 100dvh;
  display: flex;
  justify-content: center;
  align-items: center;
  transition: background-color 0.5s ease;
  overflow: hidden;
}

.app-container {
  width: 100%;
  max-width: 400px;
  height: 100%;
  max-height: 850px;
  padding: 20px;
  display: flex;
  flex-direction: column;
}

.header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 20px;
  padding: 0 10px;
}

.title {
  color: var(--text-main);
  font-weight: 700;
  font-size: 1.2rem;
  letter-spacing: -0.5px;
  transition: color 0.3s;
}

.theme-selector {
  display: flex;
  gap: 10px;
  background: var(--calc-bg);
  padding: 6px 10px;
  border-radius: 20px;
  box-shadow: 0 4px 10px var(--calc-shadow);
  transition: all 0.3s;
}

.theme-btn {
  width: 24px;
  height: 24px;
  border-radius: 50%;
  border: 2px solid transparent;
  cursor: pointer;
  transition: transform 0.2s, border-color 0.2s;
}

.theme-btn:active {
  transform: scale(0.9);
}

.theme-btn.active {
  transform: scale(1.1);
  border-color: var(--text-main) !important;
}

.calculator {
  background-color: var(--calc-bg);
  flex-grow: 1;
  border-radius: 36px;
  padding: 24px;
  display: flex;
  flex-direction: column;
  box-shadow: 0 20px 40px var(--calc-shadow), 0 1px 3px rgba(0,0,0,0.05);
  transition: all 0.5s ease;
}

.display {
  background-color: var(--display-bg);
  border-radius: 24px;
  padding: 24px;
  margin-bottom: 24px;
  display: flex;
  flex-direction: column;
  align-items: flex-end;
  justify-content: center;
  box-shadow: var(--display-shadow);
  min-height: 120px;
  transition: all 0.5s ease;
}

.history {
  color: var(--text-muted);
  font-size: 1.2rem;
  min-height: 1.5rem;
  margin-bottom: 8px;
  transition: color 0.3s;
}

.current {
  color: var(--text-main);
  font-size: 3.5rem;
  font-weight: 600;
  letter-spacing: -1px;
  word-break: break-all;
  text-align: right;
  line-height: 1;
  transition: color 0.3s;
}

.keypad {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 16px;
  flex-grow: 1;
}

.btn {
  border: none;
  border-radius: 50%;
  aspect-ratio: 1 / 1;
  font-size: 1.5rem;
  font-weight: 500;
  cursor: pointer;
  display: flex;
  justify-content: center;
  align-items: center;
  box-shadow: 0 8px 0 var(--btn-shadow-color);
  margin-bottom: 8px; /* Extra space for the thicker shadow */
  transform: translateY(0);
  transition: transform 0.05s cubic-bezier(0.4, 0, 0.2, 1), 
              box-shadow 0.05s cubic-bezier(0.4, 0, 0.2, 1),
              background-color 0.3s, color 0.3s;
}

/* tactile pressing animation */
.btn:active, .btn.pressed {
  transform: translateY(8px);
  box-shadow: 0 0px 0 var(--btn-shadow-color);
}

.btn-num {
  background-color: var(--btn-num-bg);
  color: var(--btn-num-text);
  --btn-shadow-color: var(--btn-num-shadow);
}

.btn-operator {
  background-color: var(--btn-op-bg);
  color: var(--btn-op-text);
  --btn-shadow-color: var(--btn-op-shadow);
  font-size: 1.8rem;
}

.btn-action {
  background-color: var(--btn-act-bg);
  color: var(--btn-act-text);
  --btn-shadow-color: var(--btn-act-shadow);
  font-size: 1.2rem;
}

.btn-equals {
  background-color: var(--btn-eq-bg);
  color: var(--btn-eq-text);
  --btn-shadow-color: var(--btn-eq-shadow);
  font-size: 1.8rem;
}

.btn-zero {
  grid-column: span 2;
  border-radius: 100px; /* pill shape to match the circles */
  aspect-ratio: auto;
}