That error it is because of your host server and not because of the theme. DOMDocument is a standard PHP class, you can read more about it here. If you get the error that it is not available it means your hosting provider deliberately disabled that class.
If you have root access to your server, enabling it is pretty straightforward:
For Debian, Ubuntu and other .deb based distributions:
apt-get install php5-dom
For RedHat, CentOS, Oracle Linux and other .rpm based distributions:
yum install php-xml
If you’re in a FastCGI environment, you need to restart the service after this:
service php-fpm restart
If you don’t have root access to your server you will need to ask your hosting provider to enable it for you. If you are on a shared hosting plan I doubt they will agree to do it and the only solution is to change your hosting provider.
This is the first time I’m hearing of a PHP installation where the DOMDocument class had been deliberately disabled. I have made a mental note of this and will perform some checks in version 2.0 to avoid the theme from throwing a fatal error.