To be honest if the simple theme isnt there to start with, it would be easier and quicker to re-install.
The broad steps are:
– Download RetroPie 2.3 image
– Burn with Win32 Disk Imager
– Run sudo raspi-config, and expand filesystem
– Run sudo apt-get update
– Run sudo apt-get upgrade -y
Then run the retropie setup script
and select upgrade script, and restart the setup script.
and in there you can configure your controller etc..
It should certainly include the simple theme within that process.
Check my other videos for details on scraping metadata quickly.