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Go to Settings-> User preference -> Design -> Check "Always Show design menu item"
Now right click on your chart cell and you will find an option "Custome format cell"
From here you can change the text and background color".
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That option as you have found is not available at present. The only way you could address it is with Themes as already suggested which you dont want to do or write your own piviot table extension with that property you require.