VBA Excel - activate Theme when adding new workbook (file -> new)

[jdjehanneman]

Hi,

I have added book.xltx in the xlstartup folder with the theme I want by default to be activated when opening a workbook.
This works fine but not when I open a new workbook (File->New). The new empty workbook I open doesn't have the desired default Theme activated.
I have a made an add-in toolbox which is in production for 100 users. The toolbox needs the Theme to be active at all times.

Does anyone have a solution for this?
I tried Auto Open but that doesn't solve the problem.

Private Sub Auto_Open()
    str_themepath = Application.StartupPath & "\book.xltx"
    ActiveWorkbook.ApplyTheme (str_themepath)
    'MsgBox ("Hello World!")
End Sub

Thanks in front!