Users with Microsoft Microsoft 365 Pro Plus installations on Windows 10 machines may find the custom theme/background set for MS Office clients (Outlook, Word, etc.) will revert back to default ("Colorful"). 

If this occurs, checking the following may resolve the issue:

• In Word or Outlook, go to File > Options > General and under the section Personalize your copy of MS Office check the box Always use these values regardless of sign in to Office.
• In Word or Outlook, go to File > Office Account. If you see an Account Error message/warning stating "There are problems with your account". To fix them, please sign-in again or click the Fix me button/option
• If you have installed Microsoft 365 Pro Plus on multiple machines (e.g. work, home, etc.), confirm the theme settings are the same and there isn’t a problem with the account on each installation.