body {
  font-family: serif;
}

h1, h2, h3, h4 {
  font-weight: bold;
}

.theorem, .definition, .example {
  margin: 1em 0;
}

.chapter-number::before {
  content: "Chapter ";
}

<style>
.center-line {
  text-align: center;
}
</style

.center-header {
  text-align: center;
}