Fix build of docs after sphinx update.
https://github.com/sphinx-doc/sphinx/pull/11381
https://gitlab.haskell.org/ghc/ghc/-/issues/24129
--- a/docs/users_guide/rtd-theme/layout.html
+++ b/docs/users_guide/rtd-theme/layout.html
@@ -28 +28 @@
-    <link rel="stylesheet" href="{{ pathto('_static/' + style, 1) }}" type="text/css" />
+    <link rel="stylesheet" href="{{ pathto('_static/' + styles[-1], 1) }}" type="text/css" />