Ok, isso não foi tão difícil quanto eu pensava.
Bit bagunçado porque eu não uso frequentemente o vba; mas esse código será revertido apenas para os estilos padrão:
Sub DefaultStyles()
Dim MyBook As Workbook
Dim tempBook As Workbook
Dim CurStyle As Style
Set MyBook = ActiveWorkbook
On Error Resume Next
For Each CurStyle In MyBook.Styles
Select Case CurStyle.Name
Case "20% - Accent1", "20% - Accent2", _
"20% - Accent3", "20% - Accent4", "20% - Accent5", "20% - Accent6", _
"40% - Accent1", "40% - Accent2", "40% - Accent3", "40% - Accent4", _
"40% - Accent5", "40% - Accent6", "60% - Accent1", "60% - Accent2", _
"60% - Accent3", "60% - Accent4", "60% - Accent5", "60% - Accent6", _
"Accent1", "Accent2", "Accent3", "Accent4", "Accent5", "Accent6", _
"Bad", "Calculation", "Check Cell", "Comma", "Comma [0]", "Currency", _
"Currency [0]", "Explanatory Text", "Good", "Heading 1", "Heading 2", _
"Heading 3", "Heading 4", "Input", "Linked Cell", "Neutral", "Normal", _
"Note", "Output", "Percent", "Title", "Total", "Warning Text"
Case Else
CurStyle.Delete
End Select
Next CurStyle
Set tempBook = Workbooks.Add
Application.DisplayAlerts = False
MyBook.Styles.Merge Workbook:=tempBook
Application.DisplayAlerts = True
tempBook.Close
End Sub