Keynote for Mac - Themes/ templates
How to find & download new readymade themes/ templates for Keynote presentations?
MacBook Pro 13", macOS 10.14
Mac App Store. Search for Keynote theme. There are several.
Free Keynote Templates - iWork Community (may not all be Keynote v9.1 compatible)
Presentation Templates - (some free, mixture Powerpoint and Keynote)
Presentation Zen - (the art of presentation and several other references)
Don't overlook the possibility of using non-macro Powerpoint themes/templates opened in Keynote and then saved as Keynote templates.