in the following method:

if Object.const_defined? "ActionView"
  ActionView::Base::register_template_handler 'rcss', Rcss::RcssHandler
end

this line:

if Object.const_defined? "ActionView"

returns false, even though this line:

ActionView::Base::register_template_handler 'rcss', Rcss::RcssHandler

would succeed.
I don't know why this is the case, as I don't know much about rails; just reporting the cause.