Fix theme inheritance (#24)

Search inherited themes for inherited layout.html

ntd2d/ntd2d_action/files/templates/ntd2d/layout.html

@@ -1,4 +1,4 @@
-{{% extends "{inherited_theme}/layout.html" %}}
+{{% extends "{inherited_layout}" %}}
 
 {{%- block extrahead %}}
   {{{{ super() }}}}

ntd2d/ntd2d_action/sphinxdocs.py

@@ -60,10 +60,13 @@ def epub_file(self):
     def pdf_file(self):
         return self.build_dir / "latex" / f"{self.conf.project.lower()}.pdf"
 
+    def get_theme(self, theme_name):
+        theme_factory = HTMLThemeFactory(self.sphinx_app)
+        return theme_factory.create(theme_name)
+
     @property
     def stylesheet(self):
-        theme_factory = HTMLThemeFactory(self.sphinx_app)
-        theme = theme_factory.create(self.sphinx_app.config.html_theme)
+        theme = self.get_theme(self.sphinx_app.config.html_theme)
 
         return theme.get_config("theme", "stylesheet")
 
@@ -149,6 +152,23 @@ def make_conf_file(self):
     def inherited_theme(self):
         return self.original_docs.conf.html_theme
 
+    @property
+    def inherited_layout(self):
+        """Find inherited layout.html
+
+        Inherited theme may not define its own :file:`layout.html`, but
+        rely on a theme that it, in turn, derives from.
+        `{% extends "!layout.html" %}` should work, but doesn't.
+        https://github.com/sphinx-doc/sphinx/issues/12049
+        """
+        def get_theme_layout(theme):
+            if (pathlib.Path(theme.themedir) / "layout.html").exists():
+                return f"{theme.name}/layout.html"
+            else:
+                return get_theme_layout(theme.base)
+
+        return get_theme_layout(self.get_theme(self.inherited_theme))
+
     def assimilate_theme(self, name, insert_header_footer=True):
         """Replace configuration directory with customized html theme."""
 
@@ -160,6 +180,7 @@ def assimilate_theme(self, name, insert_header_footer=True):
         self.theme = TemplateHierarchy(name=name,
                                        destination_dir=self.conf.theme_path,
                                        inherited_theme=self.inherited_theme,
+                                       inherited_layout=self.inherited_layout,
                                        inherited_css=self.stylesheet,
                                        header_footer_script=header_footer)
         self.theme.write()