PowerPoint, 
or Google Slides, themes within the original programs are very limited but there are a vast range of lovely free, downloadable themes available from Slidesmania which may be more inspiring than the office type of backgrounds typically offered.
Paula Martínez, the creator of Slidesmania, has a job that is nothing to do with education, or design, but making presentation templates has become a fun, creative hobby that she enjoys. She has shared a few, “20 Free Google Slides Templates and PowerPoint Themes’ – plus activity ideas to get you started… with ideas of how teachers could use them as a blog post, on Ditch that Textbook.
The third one is a ‘Harry Potter theme for Genius Hour’ and is for either PowerPoint or Google slides. It features crumpled paper, paper balls, light bulbs and a paper basket. She says: “Potter free presentation template for Google Slides or PowerPoint is perfect if your next lecture is about recycling ideas, or the creative process.” Ditch that Textbook have linked these to a page offering ’12 ways to really make Genius Hour work in your class.’
The tenth one is the ‘Introduce Yourself’ theme, for back to school. Paula writes: “As a mom, I would love to know more about my daughter’s teachers. So why not create a newsletter using Google Slides to send out to parents? Or even use it as a way for the students to introduce themselves to you and the rest of the class. … Lesson idea: The Introduce yourself template can be used by teachers for a back to school newsletter or presentation. Students can also create a newsletter for their parents, showing what they have learned that week.” Paula Martinez)
I like the Farm Animals Drag and Drop Shapes for little ones, the Koch Theme for space presentations and also The Wizard of Oz Template.
Paula makes the PowerPoint, or Google Slides, themes in answer to requests from people and I am sure that there are one or two available for every primary class teacher.
