1. Make sure your project has referenced this DataGridThemes project.
2. For the Window or UserControl which has the DataGrid you're working on, insert the URI to the ResourceDictionary you want into its Resources tag. For example,
<ResourceDictionary Source="/DataGridThemes;component/WhistlerBlue.xaml" />
Alternatively, you can place the ResourceDictionary reference shown above in your App.xaml file.