How do I change fonts, colors or text style for the whole book?

Click Global Settings in the top bar. It sets, for every page at once:

• the heading font and the body font
• the accent color used for rules and highlights, and the heading color
• the text style — Editorial aligns a page's words to the left edge of its photos and adds an optional title above them; Classic centers the text
• whether Auto-Arrange adds trip & event openers

Change them any time; the whole book updates. The cover, title page and dividers also have their own color schemes and fonts in the left panel when one of them is selected.