ModernWPF | small lib for creating modern

I try to implement the MVVM pattern for this sample (just simple paging part) with Microsoft.Toolkit.Mvvm unfortunately, I failed :( because I'm so noob in WPF also MVVM :))

the primary problem is how can I pass an argument of an event to command with InvokeCommandAction (Microsoft.Xaml.Behaviors.Wpf)? there is limited documentation and wiki I think... in this scenario, I change this code in MainWindow.xaml :

I'm working on a project that uses ModernWPF. I want to bind the TitleBar.Background to GetBG. Here's my code:

I have a problem with custom UI library for WPF. I use https://github.com/Kinnara/ModernWpf. So my problem is that when I ran it for the first time, this modern design appeared in my Visual Studio XAML designer. But when I opened project today after some time (or on another computer), modern UI didn't appear in Visual Studio XAML designer. I'm lost and don't know why is it happening.

Thank you very much for your answers!

I was trying to make UWP app at first, but then realized that it's too limiting for me and now trying to port existing code to classical WPF. I'm using .NET Core 3.1, Caliburn, Kinnara/ModernWpf, Microsoft.NETCore.Windows.ApiSets, Microsoft.Windows.SDK.Contracts and System.Windows.Interactivity.WPF (though, not actually using last one). So, basically, it should offered me quite smooth porting to WPF, but as I've found out, Caliburn's attached property View.Model was not working. I've tried to make my own attached property and bind it to VM's test property and it didn't worked either, that property never gets accessed and attached property's value is null.

Essencial code is following:

(This is not the real case, in real case I was using ContentPresenter in NavigationView.ContentTemplate, but this case should be the same in practice. And ofc I've tested with ContentPresenter outside and with NavigationView commented out. Also, if I'd use TextBlock for instance, instead of ContentPresenter, then binding works ok and attached property's value is as expected. What could be wrong and what makes ContentPresenter different?)

ViewModel:

I am creating a WPF application which is using ModernWpf to style everything. I am also using those two parameter on my window: