I know the question is old but it can help others, I know two ways to do it:
1st Form is Via Nuget: Right click on the project and select the "Manage NuGet Packages" option and install WPFThemes.
ItwillsettheWPF.Themesreferenceautomatically
Nowinyourmainform,maketheimagebelow
Notethatthiswaythethemewillonlybeappliedwhentheprojectisrunning
2ndForm:Downloadingthethemacodeinthelink: link
Add the thema file you downloaded to your project and within the app.xaml add the reference to it:
Notethatthiswayitloadsintodesignmodeandyouhaveaccesstothecodetoedittosuityourpreference.
Obj:Ifitdoesnotappearfirst,rebuilditinyoursolution,closeitandopenvisualstudio.