Just because the cells are blank doesn't mean the styles/formats haven't been stored with the file. You can run this procedure:
Sub ZapAllStyles()
On Error Resume Next
Application.DisplayAlerts = False
For Each x In Sheets
x.Unprotect
If Err.Number <> 0 Then
MsgBox x.Name & " is protected with a password -- unprotect & try again."
End If
Next
For i = ActiveWorkbook.Styles.Count To 1 Step -1
Application.StatusBar = i
If i Mod 10 = 0 Then DoEvents
ActiveWorkbook.Styles(i).Delete
Next
Set yy = ActiveWorkbook
Set Zz = Workbooks.Add
yy.Styles.Merge Workbook:=Zz
Zz.Close False
If Len(ActiveWorkbook.Path) > 0 Then ActiveWorkbook.Save
Application.StatusBar = False
MsgBox "Done"
End Sub