fix: roll forward fix for theme basic css import

Author: mhughes2012

AUTHORS.rst

@@ -97,6 +97,7 @@ Contributors
 * Matthew Woodcraft -- text output improvements
 * Matthias Geier -- style improvements
 * Michael Droettboom -- inheritance_diagram extension
+* Michael Hughes -- Merge inherited stylesheets in HTML themes
 * Michael Wilson -- Intersphinx HTTP basic auth support
 * Nathan Damon -- bugfix in validation of static paths in html builders
 * Nils Kattenbeck -- pygments dark style

sphinx/theming.py

@@ -80,8 +80,9 @@ def __init__(
         self.pygments_style_dark: str | None = None
         for config in reversed(configs.values()):
             options |= config.options
-            if config.stylesheets is not None:
-                self.stylesheets = config.stylesheets
+            for stylesheet in config.stylesheets:
+                if stylesheet not in self.stylesheets:
+                    self.stylesheets += (stylesheet,)
             if config.sidebar_templates is not None:
                 self.sidebar_templates = config.sidebar_templates
             if config.pygments_style_default is not None:

tests/test_theming/test_theming.py

@@ -75,7 +75,7 @@ def test_theme_api(app: SphinxTestApp) -> None:
     assert len(theme.get_theme_dirs()) == 2
 
     # direct setting
-    assert theme.get_config('theme', 'stylesheet') == 'custom.css'
+    assert theme.get_config('theme', 'stylesheet') == 'basic.css, custom.css'
     # inherited setting
     assert theme.get_config('options', 'nosidebar') == 'false'
     # nonexisting setting
@@ -169,6 +169,7 @@ def test_dark_style(app: SphinxTestApp, monkeypatch: pytest.MonkeyPatch) -> None
     assert css_file.attributes['media'] == '(prefers-color-scheme: dark)'
 
     assert sorted(str(f.filename) for f in app.builder._css_files) == [
+        '_static/basic.css',
         '_static/classic.css',
         '_static/pygments.css',
         '_static/pygments_dark.css',