Public WithEvents OutlookInspectors As Outlook.Inspectors
Public WithEvents myOlExp As Outlook.Explorer
Public WithEvents myIns As Outlook.Inspector

'startup Outlook
Private Sub Application_Startup()
    Set OutlookInspectors = Application.Inspectors
    Set myOlExp = Application.ActiveExplorer
End Sub
    
Private Sub OutlookInspectors_NewInspector(ByVal Inspector As Inspector)
    Set myIns = Inspector
End Sub

Private Sub myOlExp_InlineResponse(ByVal item As Object)

'Change the name of the style to your own
'The name is case sensitive
Dim objStyleName As String
objStyleName = "Custom Style 1"

    Dim objEditor As Word.Document
    Dim objStyle As Word.Style
    'Check if the message is Plain Text. If so - end the macro as Plain Text does not support styles
    'Bodyformat:
    '1 - plain text
    '2 - html
    '3 - rtf
    If ActiveExplorer.ActiveInlineResponse.BodyFormat = 1 Then Exit Sub
    Set objEditor = ActiveExplorer.ActiveInlineResponseWordEditor
    Set objStyle = objEditor.Styles.item(objStyleName)
    
    'No need to set the same style again - it can break copy-paste option and duplicate text
    'If you have changed style to other than defined in objStyleName program can still duplicate text
    If objEditor.Windows(1).Selection.Style = objStyle Then Exit Sub
    objEditor.Windows(1).Selection.Style = objStyle
    
    Set objEditor = Nothing
    Set objStyle = Nothing
    
End Sub

Private Sub myIns_Activate()
'Change the name of the style to your own
'The name is case sensitive
Dim objStyleName As String
objStyleName = "Custom Style 1"

    If myIns.IsWordMail() = False Then Exit Sub
    Dim objEditor As Word.Document
    Dim objStyle As Word.Style
    Dim objItem As Object
    
    Set objItem = myIns.CurrentItem
    'Check if object is empty
    If objItem Is Nothing Then Exit Sub
    
    'Check the type of the item np. Meeting request
    Dim iStringPos As Integer
    iStringPos = InStr(objItem.MessageClass, "IPM.Note")
    If iStringPos = 0 Then Exit Sub
    'If the object is flagged as sent, it means this is the uneditable content
    If objItem.Sent = "True" Then Exit Sub
    
    'Check if the message is Plain Text. If so - end the macro as Plain Text does not support styles
    'Bodyformat:
    '1 - plain text
    '2 - html
    '3 - rtf
    If objItem.BodyFormat = 1 Then Exit Sub
    If myIns.EditorType = olEditorWord Then
       Set objEditor = myIns.WordEditor
       Set objStyle = objEditor.Styles.item(objStyleName)
       If objStyle Is Nothing Then Exit Sub
    
    'No need to set the same style again - it can break copy-paste option and duplicate text
    'If you have changed style to other than defined in objStyleName program can still duplicate text
    If objEditor.Windows(1).Selection.Style = objStyle Then Exit Sub
    objEditor.Windows(1).Selection.Style = objStyle
    End If
    
    Set objEditor = Nothing
    Set objStyle = Nothing
    Set objItem = Nothing

End Sub