Describe the bug

I'm switching over from sphinx-rtd-theme to sphinx-book-theme and would like to keep some of the same behavior.
I am trying to add a header.html and footer.html under _templates folder but for some reason it's not working:

Reproduce the bug

In the footer.html, I added the attached:

And in the custom.css, I added:
/Footer/
.feedback-link {
display: inline-block;
float: left;
font-size: 16px;
}

I couldn't find any documentation either.

List your environment

No response