In Slide Master on macOS PowerPoint, it's not ordinarily possible to choose body fonts outside of a default set. In order to change this, navigate to the folder
/Applications/Microsoft\ PowerPoint.app/Contents/Resources/Office\ Themes/Theme\ Fonts
Duplicate any of the existing font themes, e.g. Arial.xml
.
Then edit the duplicate, replacing all instances of Arial with whichever font you want.
Restart PowerPoint and the dropdown list should now have a new option corresponding to the font you just added... until Office gets updated.
Note that some or all of these steps may require superuser privileges (i.e. sudo
).