2. In the Office Theme drop-down list, choose Dark Gray or Black.
3. Click the back arrow at the top left of the screen.
You should now see that the Excel interface is dark, but the spreadsheet itself, the cells on the main part of the page, are still white.
If you want the cells to be dark as well, then you need to set the background to a dark color.
1. In the bar at the top of the screen, click Page Layout, then click Background.
2. In the Insert Pictures dialog, click on the Bing search box.
3. Type “Solid Gray” or “Solid Black”, depending on how dark the background you want, and press Enter.
4. Select the background image and press Insert.
5. You may need to change the text color, if the background is too dark, the default black text will be difficult to read, so press Ctrl-A to select the entire spreadsheet, then in the ribbon, press Home, then in the Font section, press Font color drop-down list and choose white.
If the background or text color isn’t exactly what you’re looking for, you can try again until you find the right combination.