seems that read-the-docs has changed their default themes, try to fix it.

.readthedocs.yaml

@@ -30,6 +30,6 @@ sphinx:
 # Optional but recommended, declare the Python requirements required
 # to build your documentation
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
-# python:
-#   install:
-#     - requirements: docs/requirements.txt
+python:
+  install:
+    - requirements: requirements.txt

conf.py

@@ -19,10 +19,10 @@ pygments_style = 'sphinx'
 import os
 on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
 
-if not on_rtd:  # only import and set the theme if we're building docs locally
-    import sphinx_rtd_theme
-    html_theme = 'sphinx_rtd_theme'
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+# if not on_rtd:  # only import and set the theme if we're building docs locally
+import sphinx_rtd_theme
+html_theme = 'sphinx_rtd_theme'
+html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 
 # otherwise, readthedocs.org uses their theme by default, so no need to specify it
 

requirements.txt

@@ -0,0 +1 @@
+sphinx-rtd-theme==2.0.0