In numbers the tabs on my spreadsheet are green and the text is white . How can I change this. Is very difficult to read with that display

If you are running Big Sur, then go to System Preferences > General and try selecting a different Accent Color:

You could also try that in combination with change Light Mode to Dark Mode or vice versa.

SG

I am not aware of much in the system preferences for customizing the color scheme other than what SGIII showed. I have a really hard time with the gray-on-gray scheme; much more so than the white on dark green of the tabs. "Increase Contrast" (in Accessibility options) helps some with the gray-on-gray stuff if you find that to be a problem but used to cause a problem in Safari where text in the tabs would go blank. But now that you've brought this up I'm going to try it out again and see if they fixed it. Sorry it doesn't help with the tabs, though.

Edit: It does affect the tabs a little. If you select "Increase Contrast", the colors you can choose for accent color are a little darker. Still white text, though.