diff --git a/astropy_sphinx_theme/bootstrap-astropy/static/bootstrap-astropy.css b/astropy_sphinx_theme/bootstrap-astropy/static/bootstrap-astropy.css
index 1c6c649..c9577d7 100644
--- a/astropy_sphinx_theme/bootstrap-astropy/static/bootstrap-astropy.css
+++ b/astropy_sphinx_theme/bootstrap-astropy/static/bootstrap-astropy.css
@@ -237,6 +237,7 @@ div.topbar {
   overflow-x: hidden;
   overflow-y: hidden;
   width: 100%;
+  box-sizing: border-box;
 }
 
 div.topbar a.brand {