diff --git a/setup.py b/setup.py
index 714d423..0357d99 100644
--- a/setup.py
+++ b/setup.py
@@ -25,6 +25,7 @@ requirements = [
 on_rtd = os.getenv('READTHEDOCS') == 'True'
 if on_rtd:
     requirements.append('sphinxcontrib-napoleon')
+    requirements.append('Pallets-Sphinx-Themes')
 
 extra_requirements = {
     'docs': [