imitate PG style;

--- a/lib/html/isabelle.css	Fri Dec 18 15:14:59 2009 +0100
+++ b/lib/html/isabelle.css	Fri Dec 18 15:32:52 2009 +0100
@@ -26,7 +26,7 @@
 .tfree, tfree                 { color: #A020F0; }
 .tvar, tvar                   { color: #A020F0; }
 .free, free                   { color: blue; }
-.skolem, skolem               { color: #D2691E; }
+.skolem, skolem               { color: #D2691E; font-weight: bold; }
 .bound, bound                 { color: green; }
 .var, var                     { color: #00009B; }
 .numeral, numeral             { }