Unity SDK documentation fails to build on Sphinx 4+ #1989

The Unity SDK documentation does not build on a current Sphinx. sphinx-build aborts before
writing any HTML.

sphinx.errors.ThemeError: An error happened in rendering the page home.
Reason: TypeError("argument of type '_CascadingStyleSheet' is not a container or iterable")

Cause

Docs/source/_templates/layout.html uses the pre-Sphinx-4 CSS API:

{%- for css in css_files %}
{%- if css|attr("rel") %}
<link rel="{{ css.rel }}" href="{{ pathto(css.filename, 1) }}" ... />
{%- else %}
<link rel="stylesheet" href="{{ pathto(css, 1) }}" type="text/css" />

From Sphinx 4 onward css_files holds _CascadingStyleSheet objects rather than strings, so the
else branch passes an object to pathto(). AltTester® Desktop's equivalent template was already
modernised to use css_tag(); this one was not. The Unreal SDK is unaffected.

Why it matters

deploy-docs-to-live.yml uploads with rm: true, which clears the target directory before
the upload. A build producing zero pages after that step has already run would leave the published
Unity documentation empty.

docs/requirements.txt pins no versions, so which Sphinx a runner resolves is not controlled.

Setup

• Windows 11, Python 3.14.6
• Sphinx 9.1.0, recommonmark 0.7.1, sphinx-rtd-theme 3.1.0
• AltTester-Unity-SDK @ development, unmodified

Checks

• ☑️ Unmodified Docs/source builds 0 pages and aborts
• ☑️ With the template fixed, builds 20 pages
• ☑️ AltTester® Desktop docs build unaffected (27 pages, same template pattern already in use)
• ☑️ Unreal SDK docs build unaffected (19 pages)