The following macro removes the styles from the currently active workbook. If you type this code into a new workbook, please make sure to activate the workbook from which you want to reset the styles before running the macro.
Sub RebuildDefaultStyles()
'The purpose of this macro is to remove all styles in the active
'workbook and rebuild the default styles.
'It rebuilds the default styles by merging them from a new workbook.
'Dimension variables.
Dim MyBook As Workbook
Dim tempBook As Workbook
Dim CurStyle As Style
'Set MyBook to the active workbook.
Set MyBook = ActiveWorkbook
On Error Resume Next
'Delete all the styles in the workbook.
For Each CurStyle In MyBook.Styles
'If CurStyle.Name <> "Normal" Then CurStyle.Delete
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"
'Do nothing, these are the default styles
Case Else
CurStyle.Delete
End Select
Next CurStyle
'Open a new workbook.
Set tempBook = Workbooks.Add
'Disable alerts so you may merge changes to the Normal style
'from the new workbook.
Application.DisplayAlerts = False
'Merge styles from the new workbook into the existing workbook.
MyBook.Styles.Merge Workbook:=tempBook
'Enable alerts.
Application.DisplayAlerts = True
'Close the new workbook.
tempBook.Close
End Sub