Delete ALL custom cell styles EXCEL

Try this small VBA macro:

Sub StyleKiller()
    Dim N As Long, i As Long

    With ActiveWorkbook
        N = .Styles.Count
        For i = N To 1 Step -1
            If Not .Styles(i).BuiltIn Then .Styles(i).Delete
        Next i
    End With
End Sub

This resolves the Builtin vs Custom issue. Note we run the loop backwards to avoid corrupting the loop index.

To simply remove all without using INDEX, try the below:

Sub StyleKiller()
    Dim st As Style
    On Error Resume Next
    For Each st In ActiveWorkbook.Styles
      If Not st.BuiltIn Then
        st.Delete
      End If
    Next
    On Error GoTo 0
End Sub

Ok, this wasn't as hard to do as I first thought.

Bit messy as I don't often use vba; but this code will roll back to just the default styles:

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