Magento – Fatal error: Class ‘DOMDocument’ not found in app/code/core/Zend/Xml/Security.php on line 82

If you use anything like APC / Redis of other sorts of cache be sure to really empty it correctly. this is most of the times the issue on this kinds of errors. You can empty it by download the apc.php you can download it from the packages here http://pecl.php.net/package/APC

Let me know if thats solves it.

You state that in this file: app/code/local/Magentothem/Themeoptions/Helper/DATA.PHP your header looks like:

class Magentothem_Installtemplate_Helper_Data extends Mage_Core_Helper_Abstract

This is a header to another extension, not Themeoptions.

It should look like this:

class Magentothem_Themeoptions_Helper_Data extends Mage_Core_Helper_Abstract

From the original code provided which looked like:

Looking into your functions, it appears you have more hardcoded ties to this "Installtemplate" extension. An example is here:

$xmlPath = Mage::getBaseDir('code') . '/local/Magentothem/Installtemplate/Block/Xml/data_resources.xml';

This extension appears to be a mashup of two extensions. I would attempt to contact whoever you purchased this extension from, and ask for the correct code. Otherwise, I would remove the extension from your codebase altogether.