/* Reset & base */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  background: linear-gradient(to bottom right, #f0f4f8, #d9e2ec);
  color: #333;
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  padding: 1rem;
}

.container {
  background: #fff;
  padding: 2rem;
  border-radius: 16px;
  box-shadow: 0 4px 25px rgba(0,0,0,0.1);
  width: 100%;
  max-width: 400px;
  text-align: center;
}

h1 {
  margin-bottom: 1.5rem;
  font-size: 1.8rem;
  color: #2a2a2a;
}

label {
  display: block;
  font-weight: 600;
  margin-bottom: 0.5rem;
}

input[type="number"] {
  padding: 0.8rem 1rem;
  width: 100%;
  font-size: 1rem;
  border: 2px solid #d0d7de;
  border-radius: 8px;
  margin-bottom: 1.5rem;
  transition: border-color 0.3s ease;
}

input[type="number"]:focus {
  border-color: #4a90e2;
  outline: none;
}

button {
  background-color: #4a90e2;
  color: #fff;
  padding: 0.8rem 1.5rem;
  font-size: 1rem;
  border: none;
  border-radius: 10px;
  cursor: pointer;
  transition: background-color 0.3s ease, transform 0.2s ease;
  width: 100%;
}

button:hover {
  background-color: #3b7ed0;
  transform: scale(1.03);
}

#change-due {
  margin-top: 1.5rem;
  font-size: 1.1rem;
  font-weight: 600;
  background: #eaf0f6;
  padding: 1rem;
  border-radius: 10px;
  border-left: 5px solid #4a90e2;
  word-wrap: break-word;
  min-height: 60px;
}

/* Mobile adjustments */
@media (max-width: 500px) {
  h1 {
    font-size: 1.5rem;
  }

  input[type="number"] {
    font-size: 0.95rem;
    padding: 0.7rem;
  }

  button {
    font-size: 0.95rem;
    padding: 0.7rem;
  }
}