diff --git a/book-example/book/book.css b/book-example/book/book.css
index ced1eeeb..da44f461 100644
--- a/book-example/book/book.css
+++ b/book-example/book/book.css
@@ -128,6 +128,7 @@ html, body {
 }
 
 .menu-title {
+    z-index: -999;
     color: #7E7E7E;
     display: inline-block;
     font-weight: 200;
diff --git a/src/theme/book.css b/src/theme/book.css
index ced1eeeb..da44f461 100644
--- a/src/theme/book.css
+++ b/src/theme/book.css
@@ -128,6 +128,7 @@ html, body {
 }
 
 .menu-title {
+    z-index: -999;
     color: #7E7E7E;
     display: inline-block;
     font-weight: 200;