Fixes #1884: plug-in html themes cannot inherit another plug-in theme

Closes #1885: A part of code for this fix is from pull requeest #1885.
refs #1794

CHANGES

@@ -65,6 +65,8 @@ Bugs fixed
   using `any` role and `sphinx.ext.intersphinx` in same time.
 * #2121: multiple words search doesn't find pages when words across on the page title and
   the page content.
+* #1884, #1885: plug-in html themes cannot inherit another plug-in theme. Thanks to
+  Suzumizaki.
 
 Release 1.3.1 (released Mar 17, 2015)
 =====================================

sphinx/theming.py

@@ -136,9 +136,8 @@ class Theme(object):
         except configparser.NoOptionError:
             raise ThemeError('theme %r doesn\'t have "inherit" setting' % name)
 
-        if inherit in ['alabaster', 'sphinx_rtd_theme']:
-            # include 'alabaster' or 'sphinx_themes' automatically #1794
-            self.load_extra_theme(inherit)
+        # load inherited theme automatically #1794, #1884, #1885
+        self.load_extra_theme(inherit)
 
         if inherit == 'none':
             self.base = None

tests/roots/test-double-inheriting-theme/base_themes_dir/base_theme1/theme.conf

@@ -0,0 +1,2 @@
+[theme]
+inherit = basic

tests/roots/test-double-inheriting-theme/base_themes_dir/base_theme2/theme.conf

@@ -0,0 +1,2 @@
+[theme]
+inherit = base_theme1

tests/roots/test-double-inheriting-theme/conf.py

@@ -0,0 +1,7 @@
+# -*- coding: utf-8 -*-
+
+import sys, os
+
+templates_path = ['_templates']
+master_doc = 'index'
+html_theme = 'base_theme2'

tests/roots/test-double-inheriting-theme/index.rst

@@ -0,0 +1,3 @@
+============================
+Test double inheriting theme
+============================

tests/test_theming.py

@@ -14,7 +14,7 @@ import zipfile
 
 from sphinx.theming import Theme, ThemeError
 
-from util import with_app, raises, TestApp
+from util import with_app, raises, mock, path
 
 
 @with_app(confoverrides={'html_theme': 'ziptheme',
@@ -79,3 +79,19 @@ def test_js_source(app, status, warning):
     assert 'Underscore.js {v}'.format(v=v) in underscore_min, msg
     underscore_src = (app.outdir / '_static' / 'underscore-{v}.js'.format(v=v)).text()
     assert 'Underscore.js {v}'.format(v=v) in underscore_src, msg
+
+
+def test_double_inheriting_theme():
+    from sphinx.theming import load_theme_plugins  # load original before patching
+
+    def load_themes():
+        roots = path(__file__).abspath().parent / 'roots'
+        yield roots / 'test-double-inheriting-theme' / 'base_themes_dir'
+        for t in load_theme_plugins():
+            yield t
+
+    @mock.patch('sphinx.theming.load_theme_plugins', side_effect=load_themes)
+    @with_app(testroot='double-inheriting-theme')
+    def test_double_inheriting_theme_(app, status, warning, m_):
+        pass
+    yield test_double_inheriting_theme_