Single Page controller question

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I have a single page controller with:
$html = Loader::helper('html');
$this->addHeaderItem($html->css('colorbox.css'));

The CSS file is located in my theme directory. But the helper method doesn't seem to be able to find it. Things are breaking when it calls $v->getThemeDirectory(). That method returns empty when called for the single page.

I must have some file in the wrong place, but I can't seem to figure it out. This same code works fine within my block controllers. :-(