tuned color, to avoid confusion with type variables;

--- a/etc/isabelle.css	Sat Jun 25 18:24:52 2011 +0200
+++ b/etc/isabelle.css	Sat Jun 25 18:29:51 2011 +0200
@@ -19,7 +19,7 @@
 
 .hidden         { font-size: 0.1pt; visibility: hidden; }
 
-.binding        { color: #9966FF; }
+.binding        { color: #336655; }
 .entity_class   { color: red; }
 .tfree          { color: #A020F0; }
 .tvar           { color: #A020F0; }