How do I get to my Keynote 6.5 themes folder?
I found the lack of answers on Google surprising, so I had to stumble around until I found it. 

Themes are located in:

/Users/username/Library/Containers/com.apple.iWork.Keynote/Data/Library/Application Support/User Templates/

If you're not as familiar with the command line, you can open up Terminal and simply run this to get all your themes copied to your Desktop: 

cp /Users/$USER/Library/Containers/com.apple.iWork.Keynote/Data/Library/Application\ Support/User\ Templates/* ~/Desktop


Which says "copy all contents of the current users's Theme folder to that user's desktop."