It looks like you are referring to two different things here, so I will try to answer both. 
First of all, are you an instructor, or a Canvas admin? Canvas has recently added Elementary Theming (YAY!) as a feature option which can be allowed (so instructors can choose it at the course level) or turned on for all courses. (See below.)
If you are a Canvas admin, you can find this new setting in the Admin Settings in the Course Settings section. If you are an instructor, then you need to contact your Canvas admin and ask about this setting being allowed and/or turned on so you can use it.
If you would like to find ready made templates for the Elementary school level, then I would highly recommend that you check out the Canvas Commons, a repository of teacher created content which includes but is not limited to: courses, modules, pages, templates, quizzes, etc. You can filter your search by type of content as well as by grade level.
How do I share a course to Commons?
Hope this helps.
