Fix stylesheet block/inline mode

The problem was that stylesheets were being  transcluded in inline mode
(rather than block mode), which meant that triple backticks were not
being interpreted as block mode markup, but rather as a series of
single backticks.

core/ui/PageStylesheet.tid

@@ -3,7 +3,7 @@ title: $:/core/ui/PageStylesheet
 <$importvariables filter="[[$:/core/ui/PageMacros]] [all[shadows+tiddlers]tag[$:/tags/Macro]!has[draft.of]]">
 
 <$list filter="[all[shadows+tiddlers]tag[$:/tags/stylesheet]!has[draft.of]]">
-<$transclude/>
+<$transclude mode="block"/>
 </$list>
 
 </$importvariables>