How can I install template from 3rd party to Keynote?

The specific location where the files are stored is:

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

OR

You can just use the Save Theme > Add Theme to Chooser option to achieve the same result.

However, you will need to add the missing slides to your set of Master Slides manually.

Adding New Master Slides

To do that:

1. Go to a unique slide that came with the theme and copy the unique elements
2. Under View, Select Edit Master Slides
3. Make sure you can view the Navigator on the left
4. Right-click (two-finger click) over the Navigator and make a new Master Slide
5. On the new Master Slide - clear out any default elements if it's not blank
6. Paste the unique elements from the slide copied into step #1
7. Rename the new master slide to something memorable about the content
8. Exit the master slide editor (Done button) (Repeat for as many slides that you want to use with the template)

When finished you should be able to save and use the updated Keynote template normally.

Keynote used: version 8.0.1 (5579)

Missing fonts - Lots of 3rd party templates come without the fonts. You'll need to install these or choose substitutes and then reboot to have Keynote recognize all the fonts.