diff --git a/setup.cfg b/setup.cfg
index c68fda9f..288e627a 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -50,6 +50,7 @@ doc =
     jinja2<3.1.0    # to fix the issue re: jinja2
     docutils <0.17   # to fix the issue of bullet points not rendered
     sphinx_design   # for adding in-line badges etc
+    sphinx_rtd_theme    # needed by readthedocs
 tests =
     pytest
     pytest-xdist   # for running pytest in parallel