This plugin enables you to use Google Content Experiments on your WordPress site.
Please see this page.
wp_get_theme exists since WordPress 3.4.0. Try to update to the latest version of WordPress and see if that resolves the issue.
The Google Analytics code modifies the cookie domain, but this modification is not present in the experiment code. You should add some extra code to the code you get from Google.
In your Google Analytics code there is some code like
['_setDomainName', 'example.com']. You should add the following code above the experiment code you got from Google:
<script> _udn = "example.com"; </script>
In total, the experiment code you put in the Experiment Code field of the GCE plugin should look like:
<script> _udn = "example.com"; </script> <!-- Google Analytics Experiment code --> ... Contents of the experiment code ... <!-- End of Google Analytics Experiment code -->
For more info about error messages go to the Code-Checking Errors page.