```css
:root {
  --font-family: 'Arial', sans-serif;
  --bg-color-light: #f3f4f6;
  --bg-color-dark: #1e293b;
  --text-color-light: #1e293b;
  --text-color-dark: #f3f4f6;
  --primary-color: #34d399;
  --secondary-color: #3b82f6;
  --input-bg: #e2e8f0;
  --input-border: #cbd5e1;
  --button-bg: #10b981;
  --button-hover-bg: #059669;
  --transition-duration: 0.3s;
}

body {
  font-family: var(--font-family);
  margin: 0;
  padding: 0;
  background-color: var(--bg-color-light);
  color: var(--text-color-light);
  transition: background-color var(--transition-duration), color var(--transition-duration);
}

.dark-mode {
  --bg-color-light: #1e293b;
  --bg-color-dark: #f3f4f6;
  --text-color-light: #f3f4f6;
  --text-color-dark: #1e293b;
  --input-bg: #334155;
  --input-border: #475569;
  --button-bg: #047857;
}

.wrapper {
  display: grid;
  place-items: center;
  min-height: 100vh;
  padding: 1rem;
}

.calculator {
  width: 100%;
  max-width: 400px;
  background-color: var(--bg-color-dark);
  border-radius: 8px;
  box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
  overflow: hidden;
  transition: background-color var(--transition-duration);
}

.calculator-header {
  background-color: var(--primary-color);
  color: white;
  text-align: center;
  padding: 1rem;
  font-size: 1.5rem;
  font-weight: bold;
}

.calculator-body {
  padding: 1rem;
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

input[type="number"], input[type="text"] {
  width: 100%;
  padding: 0.5rem;
  font-size: 1rem;
  border: 2px solid var(--input-border);
  border-radius: 4px;
  background-color: var(--input-bg);
  transition: background-color var(--transition-duration), border-color var(--transition-duration);
}

input[type="number"]:focus, input[type="text"]:focus {
  outline: none;
  border-color: var(--primary-color);
}

button {
  width: 100%;
  padding: 0.75rem;
  font-size: 1rem;
  color: white;
  background-color: var(--button-bg);
  border: none;
  border-radius: 4px;
  cursor: pointer;
  transition: background-color var(--transition-duration), transform var(--transition-duration);
}

button:hover {
  background-color: var(--button-hover-bg);
  transform: scale(1.05);
}

.result-display {
  padding: 1rem;
  background-color: var(--input-bg);
  border: 2px solid var(--input-border);
  border-radius: 4px;
  font-size: 1.5rem;
  text-align: center;
  transition: background-color var(--transition-duration), border-color var(--transition-duration);
}

@media (prefers-color-scheme: dark) {
  body {
    background-color: var(--bg-color-dark);
    color: var(--text-color-dark);
  }

  .calculator {
    background-color: var(--bg-color-light);
  }
}

@media (max-width: 600px) {
  .calculator-header {
    font-size: 1.25rem;
  }

  input[type="number"], input[type="text"], button, .result-display {
    font-size: 0.875rem;
  }
}
```