How to refer paths relative to the theme directory?
&lt;?php print drupal_get_path('theme', 'eternal'); ?&gt;


as you've already tried, is the correct way to solve your problem. Notice though that you have to print it out as well, as drupal_get_path will only return the path, not print it for you.

I can't immediately think of any reason why it would not work in your case. Some potential suggestions to try could be


Double-check your spelling of the theme name in the code.
Clear caches.
Try disabling the theme/module and re-enable it.


Failing everything else, you'll need to debug drupal_get_filename() directly.