Hello, yvette chao.
Welcome to the Microsoft community.
I understand that you seem to be having problems adding backgrounds when using PowerPoint and Word on mobile devices. Depending on the version of Microsoft 365 and Office / Slideshow / Home / iPhone or iPad where the issue is posted, a note of clarification is in order: the mobile versions of the Office apps may differ in functionality from the desktop versions, and some features may not be available. Additionally, in order to more accurately determine the issue you are experiencing, I need to confirm more information with you:
Are you using the Word/PowerPoint **** apps **** or the web side? If it's an application, combined with my actual testing and information gathering, here are the specific conclusions I've come to:
The Word app on iPhone/iPad does not support adding backgrounds to documents. Because Word is a professional word processing software, its main function is to process text and formatting, and adding backgrounds is not its main function, so if you want to add backgrounds to your documents, please use the Word app on your computer.
But after my test: you can create a new word document > click more (three dots icon in the upper right corner), to achieve the dark and light background switch (or iPad: through the view > switch background to achieve).
**>**On iPad: with the "Design" button, you can add a background by going to Design->Set Background Format. If the "Design" button does not appear on your device, I suggest you share a screenshot of how you are using it so that I can visualize what you are experiencing.
**>**On iPhone: there is no "Design" button, but you can change the theme by clicking Edit -> Layout.
I hope the above information is helpful and should you have any questions or require clarification, do not hesitate to reach out to me. Looking forward to your reply.
Best Regards,
Hannah.Z- MSFT |Microsoft Community Support Specialist