diff --git a/doc/readthedocs_requirements.txt b/doc/readthedocs_requirements.txt
index d93014710..ca4b4e69b 100644
--- a/doc/readthedocs_requirements.txt
+++ b/doc/readthedocs_requirements.txt
@@ -6,3 +6,4 @@
 -r ../test-requirements.txt
 -r ../extra-requirements.txt
 -r ./requirements.txt
+sphinx_rtd_theme>=0.5.1       # MIT
diff --git a/doc/requirements.txt b/doc/requirements.txt
index 3dee1be8a..70f78ad10 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -4,4 +4,3 @@
 
 reno>=3.2.0                   # Apache-2.0
 sphinx>=3.4.2                 # BSD
-sphinx_rtd_theme>=0.5.1       # MIT
diff --git a/doc/source/conf.py b/doc/source/conf.py
index ef695d13a..58134afa0 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -97,7 +97,11 @@ todo_include_todos = True
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 #
-html_theme = "sphinx_rtd_theme"
+try:
+    import sphinx_rtd_theme
+    html_theme = "sphinx_rtd_theme"
+except ModuleNotFoundError:
+    html_theme = 'alabaster'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
diff --git a/releasenotes/source/conf.py b/releasenotes/source/conf.py
index 2d1dd5c7c..8610bd927 100644
--- a/releasenotes/source/conf.py
+++ b/releasenotes/source/conf.py
@@ -81,7 +81,7 @@ master_doc = 'index'
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = 'en'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
@@ -97,25 +97,28 @@ pygments_style = 'sphinx'
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 #
-html_theme = "sphinx_rtd_theme"
+try:
+    import sphinx_rtd_theme
+    html_theme = "sphinx_rtd_theme"
+    # Theme options are theme-specific and customize the look and feel of a
+    # theme further. For a list of options available for each theme, see the
+    # documentation.
+    html_theme_options = {
+        "logo_only": False,
+        "display_version": True,
+        "prev_next_buttons_location": "top",
+        "style_external_links": True,
+        # Toc options
+        "collapse_navigation": True,
+        "sticky_navigation": True,
+        "navigation_depth": 4,
+        "includehidden": True,
+        "titles_only": False,
+    }
+except ModuleNotFoundError:
+    html_theme = 'alabaster'
+    html_theme_options = {}
 
-# Theme options are theme-specific and customize the look and feel of a theme
-# further.  For a list of options available for each theme, see the
-# documentation.
-#
-html_theme_options = {
-    "canonical_url": "https://docs.openstack.org/tobiko/latest/",
-    "logo_only": False,
-    "display_version": True,
-    "prev_next_buttons_location": "top",
-    "style_external_links": True,
-    # Toc options
-    "collapse_navigation": True,
-    "sticky_navigation": True,
-    "navigation_depth": 4,
-    "includehidden": True,
-    "titles_only": False,
-}
 
 
 # Add any paths that contain custom static files (such as style sheets) here,