To apply your PowerPoint Theme to Excel, please follow the instructions:
- Save the current PPT Theme in Design > Theme > Save Current Theme > Save as type: Office Theme
- Open a blank workbook in Excel
- Go to Page Layout > Theme > and select the theme you saved in "Custom"
- Save this excel document using the right theme as an Excel Template in File > Save as > Save as type: Office Theme in "Custom Office Template" folder
You can find and use this template afterward in New > Personal.