:root {
    --primary: #20c997;
    --teal-hover: #138d75;
    --navy: #001f3f;
    --dark-navy: #001737;
    --bg-light: #f4f4f4;
    --white: #ffffff;
    --muted: #aaa;
    --dark-bg: #1f1f1f;
    --dark-card: #2c2c2c;
    --light-text: #f0f0f0;
  }
  
  * {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-family: 'Segoe UI', sans-serif;
  }
  
  body {
    background: var(--bg-light);
    color: var(--navy);
    padding: 2rem;
    transition: background 0.3s, color 0.3s;
  }
  
  body.dark {
    background: var(--dark-bg);
    color: var(--light-text);
  }
  
  .container {
    max-width: 500px;
    margin: auto;
    background: var(--white);
    padding: 2rem;
    border-radius: 1rem;
    box-shadow: 0 4px 12px rgba(0,0,0,0.1);
    transition: background 0.3s;
  }
  
  body.dark .container {
    background: var(--dark-card);
  }
  
  .brand-title {
    font-size: 1.8rem;
    text-align: center;
    margin-bottom: 2rem;
    font-weight: bold;
    background: linear-gradient(90deg, #20c997, #001f3f);
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
  }
  
  label {
    display: block;
    margin-bottom: 1rem;
    font-weight: 500;
  }
  
  input[type="number"], select {
    width: 100%;
    padding: 0.6rem;
    margin-top: 0.3rem;
    border-radius: 8px;
    border: 1px solid var(--muted);
    background: var(--white);
    transition: background 0.3s;
  }
  
  body.dark input, body.dark select {
    background: #333;
    color: var(--light-text);
    border-color: #444;
  }
  
  .tip-options {
    display: flex;
    flex-wrap: wrap;
    gap: 0.5rem;
    margin-top: 0.5rem;
  }
  
  .tip-btn {
    flex: 1 0 20%;
    padding: 0.6rem;
    background: var(--primary);
    color: #fff;
    border: none;
    border-radius: 8px;
    cursor: pointer;
    transition: background 0.2s;
  }
  
  .tip-btn:hover {
    background: var(--teal-hover);
  }
  
  .results {
    margin-top: 1.5rem;
    font-size: 1.1rem;
  }
  
  #resetBtn {
    margin-top: 1rem;
    background: var(--navy);
    color: #fff;
    padding: 0.8rem;
    width: 100%;
    border: none;
    border-radius: 8px;
    font-weight: bold;
    cursor: pointer;
    transition: background 0.3s;
  }
  
  #resetBtn:hover {
    background: var(--dark-navy);
  }
  
  .toggle {
    text-align: center;
    margin-top: 1.5rem;
  }
  
  .toggle input {
    margin-left: 0.5rem;
  }
  
  @media (max-width: 500px) {
    .tip-options {
      flex-direction: column;
    }
  
    .tip-btn {
      flex: 1 0 100%;
    }
  }