Excel and Word take their background color from Fonts, Colors, & System Metrics > Fonts & System Metrics > WINDOW.
Maybe you can use this guide to force separate Excel and Word themes:
https://support.office.com/en-us/article/Change-a-theme-and-make-it-the-default-in-Word-or-Excel-c846f997-968e-4daa-b2d4-42bd2afef904I'm not sure because I don't use either of them.