It's not CSS you want to modify, but your HTML file. Instead of
Code:
<p>CHAPTER 10</p>
<p>Blablabla...</p>
you want to use:
Code:
<h2>Chapter 10</h2>
<p>Blablabla...</p>
(The number in H2 can be anything from 1 to 6, 1 being the largest and most important.)
If you then want to enlarge the heading even more, you can use CSS:
Code:
h2 { font-size: 200%; }