Theme Fonts comprise a pair of font choices, one each for your slide titles (Heading font) and the other for everything else on your slides (Body font). You can use existing Theme Fonts available in PowerPoint or even create them on your own. Additionally, you can opt to edit existing Theme Font pairs (sets).
To learn more, choose your version of PowerPoint. If we do not have a tutorial for your version of PowerPoint, explore the version closest to the one you use.