I had some trouble when moving around templates with the print themer_body_class(); snippet made in template.tpl.php files and the themer module not installed.
In fact the function was called from the template and the module not installed.
So I suggest to use the following code in the template.
If you agree with me per aps, you can update the README.txt

<body<?php print $body_attributes; ?> <?php if (function_exists ('themer_body_class')) {print themer_body_class();
?> >

This way you can safely leave the code in the template without Themer being installed.