Turn dark mode on or off

1. Select the File menu and select Options.

2. On the General tab, under Personalize your copy of Microsoft Office, select the Office Theme drop-down arrow and select an option.

Excel's dark mode is reliant on MacOS settings. To check that you have dark mode enabled:

1. From the  Apple icon, select System Settings and choose Appearance,

2. Select Dark or Auto.

From the Excel menu, 

1. Select Preferences and choose General.

2. Select or clear Turn off Dark Mode.

Note: If Dark mode is on for they system, this turns it on or off for Excel

To toggle dark mode on or off, on the View menu, select Dark Mode.