A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Hello there,
Sorry to hear you're experiencing issues with regards to the Office 365 borders and hope I can help you change it back with the following steps:
1)Open a Microsoft 365 application, like Word, Excel, or PowerPoint.
2)In the top left, select File > Options > General > Office Theme.
3)Choose your favorite option.
These steps was referenced from the following source and for further clarification please visit: https://support.microsoft.com/office/63e65e1c-08d4-4dea-820e-335f54672310
I hope these following steps help fix/address your issue and should you have any further problems/concerns please do not hesitate to contact me again.
Thanks,
Parth P.