How can I change the theme (red) color of PowerPoint for Mac?
There is no option in MS Office to do this, but how about Mac system?


There is no built-in option in macOS to theme/visually style apps. The most you can do is to turn on Dark Mode by going to System Preferences → General and select Dark for Appearance (1st item at the top). This way, you can get dark grey appearance in PowerPoint for macOS.

Office for Mac version 16.20, Office 365 and Office 2019 support Dark Mode on macOS Mojave. From this linked article on MacRumors:


Microsoft Office Apps Now Support Dark Mode on macOS Mojave



  Microsoft today updated its suite of Office for Mac apps, including Word, Excel, PowerPoint, and Outlook, with support for Dark Mode on macOS Mojave and several other new features. The updates are available to both Office 365 subscribers and to users who purchased Office 2019 for Mac as a one-time purchase.
  
  


and


November 2018 release



  Microsoft Auto Update 4.5
  
  
  Support for Dark Mode
  


and


https://twitter.com/derek_snook/status/1072614131730169858



  And with v16.20 some customers will see our Ribbon Visual Refresh with new icons and styling in Office 365 for Mac, as we begin rolling out and getting your feedback! On macOS Mojave, the new icons adapt to Light and Dark Mode as well!  #Office365
  
   


To learn more about Dark Mode, refer to the Apple Support article:


How to use Dark Mode on your Mac