Watch small tutorial video http://www.youtube.com/watch?v= 67LwOJeilvA

1) Add theme project to your solution
2) Paste code in App.xaml:
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <!-- Reference main style dictionary-->
                <ResourceDictionary Source="pack://application:,,,/HR.Themes.Fischer.WPF;component/Style.xaml"/>
                <!-- Reference color style -->
                <ResourceDictionary Source="pack://application:,,,/HR.Themes.Fischer.WPF;component/Colors/Colors_Blue.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>