The file you're opening does not have any CSS applied to it. The index.html gives you the structure of your book, but there are additional files that add styling and interactivity (usually CSS and Javascript).
You can see what this webpage looks like as raw HTML (no CSS/JS) below.
I realized that when I deploy those book example using netlify, it looks as I wanted.
I don't know much about CSS and how it works behind rmarkdown, but anyway there is no problem at this time.
Thank you.