Change workbook themes

To change the theme of a workbook in Excel, follow these steps:
1. Open the workbook that you want to change the theme for.
2. Click on the "Page Layout" tab on the ribbon at the top of the Excel window.
3. Click on the "Themes" dropdown in the "Themes" section of the ribbon.
4. Select the theme you want to apply to the workbook from the dropdown menu.
5. The theme will be applied to the entire workbook, including all sheets and charts.
You can also create your own custom theme by selecting "Create New Theme Colors" from the "Themes" dropdown and choosing your own colors for the various elements of the theme, such as the background, text, and accents.