Application.OpenThemeFile (PowerPoint)

Opens the specified theme file (*thmx).

OpenThemeFile (themeFileName)

themeFileName: The path of the theme file (*.thmx) to open.

Dim strthemeFileName As String: strthemeFileName = 
Dim thmOpenThemeFile As Theme
Set thmOpenThemeFile = Application.OpenThemeFile(themeFileName:=strthemeFileName)