Reinstated the RTD Sphinx Theme for documentation

Sometime in 2023 the default theme on Read-the-Docs was changed to
Alabaster. This change enforces the old theme (sphinx_rtd_theme).

docs/conf.py

@@ -30,6 +30,7 @@ on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 extensions = [
+        "sphinx_rtd_theme"
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -41,8 +42,7 @@ templates_path = ['_templates']
 exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store']
 
 
-if not on_rtd:
+html_theme = 'sphinx_rtd_theme'
-    html_theme = 'sphinx_rtd_theme'
 
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,