The cells color theme

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I hope that you won't ever need to print out a worksheet - it would use up a lot of toner!

Create a blank workbook and modify the Normal style to have a black background and white font.

Save this workbook as an Excel template (.xltx) named Book.xltx in Excel's startup folder (by default %AppData%\Microsoft\Excel\XLSTART). The name Book.xltx is mandatory for the English version of Excel; it is language-dependent - for example, in French it is Livre.xltx.

Also create a version of this template with just one worksheet and save it as a template named Sheet.xltx in the same folder. The name is mandatory again, but language-dependent; in French it would be Feuil.xltx).

Excel will use these when you press Ctrl+N or click the New button on the Quick Access Toolbar to create a new workbook, and when you click the + to insert a new worksheet.