helper not found fa

Problem

it looks like there's an undocumented dependency on a fontawesome helper.

I wanted to change a page to say the meta tag robots noindex should exist, regardless of print.

Steps

1. create theme folder
2. copy index.hbs
3. run a build

Possible Solution(s)

either the web docs are broken, the template in source is, there's a missing dependency, or I'm missing something. It's possible I'm missing some steps on overriding the theme.

Notes

mdbook build
2025-09-12 21:08:52 [INFO] (mdbook::book): Book building has started
2025-09-12 21:08:52 [INFO] (mdbook::book): Running the html backend
2025-09-12 21:08:52 [ERROR] (mdbook::utils): Error: Rendering failed
2025-09-12 21:08:52 [ERROR] (mdbook::utils): Caused By: Error rendering "index" line 146, col 29: Helper not found fa
2025-09-12 21:08:52 [ERROR] (mdbook::utils): Caused By: Helper not found fa
make: *** [Makefile:3: build] Error 101

Version

mdbook v0.4.52

Start with the copied index.hbs around line 146, then inspect the source template it overrides and the Makefile build entry point. Run mdbook build while comparing the theme override steps with the web documentation. Done means the documented override workflow renders successfully without the missing fa helper error.