Thanks for using Syncfusion product.
From your update, we have noted that you are trying to update the Office365 style using merged dictionaries by providing path. But, our Office365 style has been available in “Synfusion.Themes.Office365.WPF” assembly and it is not available under Tool.WPF, so you are facing issue while applying style. Now, we have prepared a simple sample in which we have added “Syncfusion.SfSkinManager.WPF” and “Syncfusion.Themes.Office365.WPF” references and override Office365 style using merged dictionaries and it can be downloaded from following location.
Please check with the sample and let us know whether your requirement has been achieved.