Setting theme in WPF Application

Hi,

I am trying to make UI of my WPF project better by adding themes. I downloaded the themes using Nuget Manager, and added one of them in the following manner in :

<ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionarySource="Themes/BureauBlack.xaml"/>
        </ResourceDictionary.MergedDictionaries>
</ResourceDictionary>

But when I run the project, there is not change in the look. Adding screenshot of wpf after adding theme.

The project is a vsix, in that I have added wpf.