diff -r e3dc9ea67a62 -r d07300e8a14d src/Pure/Thy/html.scala
--- a/src/Pure/Thy/html.scala	Thu Jun 01 21:15:56 2017 +0200
+++ b/src/Pure/Thy/html.scala	Thu Jun 01 21:24:33 2017 +0200
@@ -225,14 +225,16 @@
 
   /* document directory */
 
+  def isabelle_css: Path = Path.explode("~~/etc/isabelle.css")
+
   def init_dir(dir: Path)
   {
     Isabelle_System.mkdirs(dir)
-    File.copy(Path.explode("~~/etc/isabelle.css"), dir)
+    File.copy(isabelle_css, dir)
   }
 
   def write_document(dir: Path, name: String, head: XML.Body, body: XML.Body,
-    css: String = "isabelle.css", hidden: Boolean = true)
+    css: String = isabelle_css.base.implode, hidden: Boolean = true)
   {
     init_dir(dir)
     File.write(dir + Path.basic(name), output_document(head, body, css = css, hidden = hidden))