```css
/* Modern Light Theme for Count Up Timer */
:root {
  --primary-color: #4263eb;
  --secondary-color: #748ffc;
  --text-color: #333;
  --light-text: #666;
  --background: #f7f9fc;
  --panel-bg: #ffffff;
  --shadow: 0 8px 20px rgba(0, 0, 0, 0.08);
  --border-radius: 12px;
  --transition: all 0.3s ease;
}

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: 'Ubuntu', sans-serif;
  background-color: var(--background);
  color: var(--text-color);
  line-height: 1.6;
  padding: 2rem 1rem;
  max-width: 100%;
  overflow-x: hidden;
}

#stopWatch {
  background-color: var(--panel-bg);
  max-width: 500px;
  margin: 0 auto 3rem;
  padding: 2rem;
  border-radius: var(--border-radius);
  box-shadow: var(--shadow);
  text-align: center;
}

h5 {
  font-size: 1.8rem;
  margin-bottom: 0.5rem;
  color: var(--primary-color);
  font-weight: 600;
}

h6 {
  font-size: 0.9rem;
  color: var(--light-text);
  margin-bottom: 1.5rem;
  font-weight: 400;
}

#timer {
  font-size: 3.5rem;
  font-weight: 300;
  margin: 1rem 0 2rem;
  color: var(--text-color);
}

button {
  background-color: var(--primary-color);
  color: white;
  border: none;
  padding: 0.8rem 1.5rem;
  margin: 0 0.5rem 1rem;
  border-radius: 2rem;
  cursor: pointer;
  font-family: 'Ubuntu', sans-serif;
  font-size: 1rem;
  transition: var(--transition);
  outline: none;
}

button:hover {
  background-color: var(--secondary-color);
  transform: translateY(-2px);
}

button:active {
  transform: translateY(0);
}

#start {
  background-color: #12b886;
}

#start:hover {
  background-color: #20c997;
}

#stop {
  background-color: #fa5252;
}

#stop:hover {
  background-color: #ff6b6b;
}

#pause, #continue {
  background-color: #fab005;
}

#pause:hover, #continue:hover {
  background-color: #fcc419;
}

.fulltime {
  font-size: 1rem;
  color: var(--light-text);
  margin-top: 1rem;
}

.article {
  max-width: 800px;
  margin: 0 auto;
  background-color: var(--panel-bg);
  padding: 2.5rem;
  border-radius: var(--border-radius);
  box-shadow: var(--shadow);
}

.article p {
  margin-bottom: 1.5rem;
  color: var(--text-color);
}

.article strong {
  color: var(--primary-color);
}

.article ul {
  margin-bottom: 1.5rem;
  padding-left: 1.5rem;
}

.article li {
  margin-bottom: 0.5rem;
}

/* Responsive adjustments */
@media (max-width: 600px) {
  #stopWatch {
    padding: 1.5rem;
  }

  #timer {
    font-size: 2.5rem;
  }

  button {
    padding: 0.7rem 1.2rem;
    font-size: 0.9rem;
    margin: 0 0.3rem 0.8rem;
  }

  .article {
    padding: 1.5rem;
  }
}
```