Before upgrading to version 4.5 I want to export my theme HTML templates because I've customized them quite a bit and I'm not totally sure where all the customizations are located within each template. 

Question: Is it possible to export these templates so I can look them over offline and compare with stock templates?

There is a built in tool in the edit themes section in which you can compare to a stock template to see your exact changes and update as needed.  No need to download them in this case as that feature is all built in.

Select edit HTML& CSS on a theme in the admincp > from the upper right drop down menu select "Show Default"  and it will show the two side by side, highlighting the changes between the two templates. 

Lovely. Thanks. 

This is a fantastic feature!