Hello @akorn
Please check this topic, we discussed similar question: Permanently change default font
You have to replace template for pptx file with your one.
We are working on ‘Master Slide’ feature already (internal number - 45381). I will update this post when we have something to share about this feature.
We have released version 8.2 of the Document Server where we added the master slide feature to our presentation editor. If you are using ONLYOFFICE Desktop Editors app, the new version is going to be released in about one week.