Excel banner goes grey in MacBook when screen is maximised

This is a known Excel for Mac display/appearance bug, especially on macOS Ventura/Sonoma and Office 365 builds from the last year.
You’re not doing anything wrong — the grey banner appears only in full-screen mode (when clicking the green traffic-light button), and the ribbon/icons become low-contrast.

Workarounds you can try…

Update Excel

Microsoft patched this in several builds but the bug still reappears:

Excel → Help → Check for Updates

Install the latest version.

Don't use full-screen — use “zoom” instead

Instead of the green full-screen button:

1. Hold Option
2. Click the green button (it turns into a “+” symbol) → this zooms the window to fill most of the screen but avoids full-screen mode.

Change Office theme

Go to:

Excel → Preferences → General → Personalize
Try switching between:

• Colorful (default)
• Classic
• Dark Mode

Sometimes switching themes resets the ribbon colouring.

Disable macOS automatic dark mode

If macOS is using Auto (light/dark) mode:

System Settings → Appearance → Light

Excel often misbehaves in Auto mode in full-screen.

Turn off “Reduce Transparency” (macOS)

Go to:

System Settings → Accessibility → Display
Turn off: Reduce Transparency

Excel sometimes misinterprets this when switching UI modes.

I hope it helps them.