diff --git a/book/_config.yml b/book/_config.yml
index e3c2dfc..cb1ac1c 100644
--- a/book/_config.yml
+++ b/book/_config.yml
@@ -23,6 +23,7 @@ sphinx:
   - sphinx_named_colors
   - sphinx_iframes
   - sphinx_prime_applets
+  - sphinx_dropdown_toggle
   config:
     html_js_files:
     - https://cdnjs.cloudflare.com/ajax/libs/require.js/2.3.4/require.min.js