Visual reviews and mkdocs

I’m wondering whether anyone else has managed to get visual reviews working with mkdocs? I wrote a script to inject the header into the generated HTML (this is for mkdocs-material):

#!/bin/sh

# Guess the MR IID.
echo "${CI_OPEN_MERGE_REQUESTS}" >mr.txt
MR_IID=$(awk  -F'[!,]' '{print$2}' mr.txt)

# Inject HTML into the correct Jinja2 template, overriding the default theme.
echo "Creating overrides directory."
mkdir -p overrides
echo "Overriding default material theme with review app extensions."
cat > overrides/main.html << HTML
{% extends "base.html" %}
{% block extrahead %}
<script defer
    data-project-id='${CI_PROJECT_ID}'
    data-project-path='${CI_PROJECT_PATH}'
    <!-- Remove the following line to use the same script for multiple merge requests -->
    data-merge-request-id='${MR_IID}'
    data-mr-url='https://gitlab.com'
    id='review-app-toolbar-script'
    src='https://gitlab.com/assets/webpack/visual_review_toolbar.js'></script>
{% endblock %}
HTML

# Add the theme overrides to the mkdocs configuration file.
cat > custom.yml << YAML
    # Used for GitLab review apps.
    custom_dir: overrides
YAML
echo "Injecting new configuration to the mkdocs.yml file."
sed -i "/    name: 'material'/r custom.yml" mkdocs.yml

and the HTML does seem to appear in the <head> element of the pages, but the review app doesn’t look any different to the site in a development environment.

Have I missed something?

TIA,

Sarah