Changing your background isnt supported in. You could change your documents to have a black background and white text, but those colors would be part of each document you save. On Mac or Windows, you can use virtual backgrounds in meetings and events using the Webex Meetings desktop app. It’s worth noting that documents you create while in your chosen dark mode will still have a white background and black text by default. Click the Apple menu in the top left, select ‘System Preferences’, then ‘General’ and at the top of the dialog box you can switch your macOS from Light to Dark. To put Office in dark mode on Mac, you’ll need to change this setting in the macOS system preferences. To enable dark mode, select ‘Black’ for the darkest possible style or ‘Dark Gray’ for more of a charcoal feel. The default theme is ‘Colorful’ (which keeps your Excel green, your PowerPoint red etc.) but you can also select ‘White’ for brighter whites across Office applications. Click the ‘Account’ option in the sidebar, then the ‘Office Theme’ dropdown menu to select your desired theme.
To change your theme, click the ‘File’ menu at the top left corner of an Office application like Word, Excel, Outlook, or PowerPoint.