diff --git a/book.toml b/book.toml
index 6b47d7f..4814397 100644
--- a/book.toml
+++ b/book.toml
@@ -13,7 +13,6 @@ port = 3001
 mathjax-support = true
 no-section-label = true
 fold = { enable = true, level = 1 }
-additional-css = ["custom.css"]
 additional-js = ["custom.js"]
 section-anchors = false