This Powershell Script will revert all of the default fonts in the O365 apps back to Calibri for all users . This includes Outlook new and old, Powerpoint, Excel, and Word.
Create a folder called BackToCalibri on the C:\ drive Put your Blank.potx and Normal.dotm in there.
The script will run without these, though powerpoint & Word won't configure
Then simply run the script as administrator, and it will take care of the rest
.\BackToCalibri.ps1
Sidenote: You can change the Blank.potx to any template if your org has a nice one that you would like to use
Log into your target organization's Exchange Online Shell
Run the following command to change the font for all users
Get-Mailbox -ResultSize unlimited | Set-MailboxMessageConfiguration -DefaultFontName "Calibri" -DefaultFontSize "10"
https://www.pdq.com/blog/modifying-the-registry-users-powershell/
https://github.com/j0eyv/ProactiveRemediations/blob/main/Remediate_OutlookFont_Calibri.ps1
https://www.shapechef.com/blog/change-the-default-font-in-powerpoint