Hello Jo,
Thank you for reaching out to the Microsoft Community. My name is Marisol and I'm happy to help.
Based on your description, I can see that you are trying to change the appearance of Excel. Can you confirm if you are referring to theme?
If so, please try these steps:
To change the color of your Microsoft 365 apps:
If you're on Windows:
- Open Excel
- In the top left, select File > Options > General > Office Theme.
- Choose your favorite option.
Please note that based on the image you provided, please see More... (which is next to Account). You should be able to find Options after selecting More...
To see full information with screenshot, please refer to this link: https://support.microsoft.com/office/63e65e1c-08d4-4dea-820e-335f54672310
Let me know if you need more help.
Regards, Marisol D.