How to make MS Excel cells display dark background with white text instead of the reverse which is default?

This updated code makes all sheets black and adds cell borders to simulate the grid lines;

Private Sub Workbook_Open()

    Dim xsheet As Worksheet

    For Each xsheet In ThisWorkbook.Worksheets
        xsheet.Select
        With Range(Cells(1, 1), Cells(1048576, 5000))
            .Interior.Color = vbBlack 'xlNone '
            .Font.Name = "Calibri"
            '.Font.Size = "11"
            .Font.Color = vbWhite 'vbBlack
            .Borders.LineStyle = xlContinuous
            .Borders.Weight = xlThin
            .Borders.ColorIndex = 49 'colors here http://dmcritchie.mvps.org/excel/colors.htm
        End With
    
    Next xsheet

End Sub

The most easiest method I can suggest you is VBA (Macro).

Private Sub Workbook_Open()

Dim xsheet As Worksheet 

For Each xsheet In ThisWorkbook.Worksheets

ThisWorkbook.Sheets.Select
    With Range(Cells(1, 1), Cells(1048576, 5000))
        .Interior.Color = vbCyan
        .Font.Name = "Calibri"
        .Font.Size = "11"
        .Font.Color = vbWhite
    End With
    Next xsheet
End Sub

How it works:

1. Press Alt+F11 to open VB editor.
2. Find & Click the This Workbook Icon.
3. Select Workbook from left Dropdown & Open from right Dropdown.
4. Copy & Paste lines from ThisWorkbook.Sheets.Select to End With & Save the Workbook.

Next time when you open the Workbook, you find the new look.

Note, Sheet's dimension, Interior & Font Color as well as Font name & size are editable.