Is there more Layout Templates or Themes we can get with Omeka Classic other than the default?
All of the publicly shared themes for Omeka Classic are available for download and installation: http://omeka.org/classic/themes
We always welcome the submission of themes from community designers. So, if you see a layout or theme that you like that isn’t available in the repository, please encourage that site to submit their work.
After I download them, how do I apply them to our site?
You’ll have to install them on your server.
The Omeka Classic documentation has some basic instructions: Working with Plugins - Omeka Classic User Manual
To add themes to your installation, you might want to check the instructions at Themes - Omeka Classic User Manual
For additional themes, Daniel compiled a very long list at Omeka Themes | From Omeka Classic to Omeka Semantic
Hope this helps.