Theme Colors cannot be the full name of the template. It looks like a folder to me. If you are trying to apply a COLOR SCHEME from this folder you will need different code. You need to state CLEARLY the name you are trying to apply including the extension (.xml for a color scheme, .potx or .thmx for a template / theme)

ActivePresentation.SlideMaster.Theme.ThemeColorScheme.Load (Path to xml color scheme)