Custom Checkbox control in Excel

Sorry not .Shadow, but .Fill (see code below)

I also tried to hide the cell's value (true or false) with editing the font color. Better is to control it with conditional formatting while formatting the cell's background color.

Sub Rectangle_Toggle(Button As Variant)
    With Application.ActiveSheet.Shapes(Button)
    
        'Transparency of the shape 1 = transparent, 0 = solid
        .Fill.Transparency = 1
        'Hides the cell's value
        Range(.TopLeftCell.Address).Font.Color = Range(.TopLeftCell.Address).Interior.Color
        
        If .TextFrame.Characters.Text = "X" Then 'Chr$(252) Then
         .TextFrame.Characters.Text = ""
         .Fill.ForeColor.RGB = RGB(255, 255, 255)
         Range(.TopLeftCell.Address).Value = False
        Else
        .TextFrame.Characters.Text = "X" 'Chr$(252)
        .Fill.ForeColor.RGB = RGB(46, 208, 80)    '(50, 195, 50)  <-- change your color here
        Range(.TopLeftCell.Address).Value = True
        End If
    End With
End Sub