Theme path/url not available - broken images.

I'm doing some work with this plugin and found that none of the images are showing up. After tracing through the code, I find that $theme_url and $theme_path are declared in the HTML_Emailer object, but the values are never more than an empty string. This prevents the header images from displaying. I thought I'd just append the theme name onto those variables, but the theme name is nowhere to be found. So, question: where to get the theme name, so I can set it within the object? Example code would be appreciated!

To clarify:

Template Path/URL
$this->template_directory = /site/wp-content/.../htmlemail/lib/templates/
$this->template_url = http://site.com/wp-content/.../htmlemail/lib/templates/

Theme Path/URL (Missing!)
$this->theme_path = /site/wp-content/.../htmlemail/lib/templates/Mytemplate/
$this->theme_url = http://site.com/wp-content/.../htmlemail/lib/templates/Mytemplate/