The default Page.ss template that the "Simple" theme includes breaks the highlighting, specifically this line:
<body class="$ClassName.ShortName<% if not $Menu(2) %> no-sidebar<% end_if %>" <% if $i18nScriptDirection %>dir="$i18nScriptDirection"<% end_if %>>
Attaching image of how it breaks:
The default Page.ss template that the "Simple" theme includes breaks the highlighting, specifically this line:
Attaching image of how it breaks: