proper theory name, e.g. for HTML/PIDE presentation;

--- a/src/Pure/theory.ML	Tue Jan 05 16:24:59 2021 +0100
+++ b/src/Pure/theory.ML	Tue Jan 05 16:39:53 2021 +0100
@@ -132,7 +132,7 @@
 
 fun get_markup thy =
   let val {pos, id, ...} = rep_theory thy
-  in theory_markup false (Context.theory_name thy) id pos end;
+  in theory_markup false (Context.theory_long_name thy) id pos end;
 
 fun check long ctxt (name, pos) =
   let