Something like:
h1, h2, h3, h4, h5, h6 {
font-size: 2rem;
}
And you can change it for specific text if you can work out a specific CSS selector for it. Then for the third question, something like:
.course-content h1,
.course-content h2,
.course-content h3,
.course-content h4,
.course-content h5,
.course-content h6 {
font-size: 2rem;
}