This looks like a Visual Studio designer issue. Can you compile and deploy it to confirm that the styles are respected at runtime?
Generally speaking, the Xamarin.Forms designer is unreliable (I personally always have it disabled). Microsoft has been working on it, but it's not like WPF or UWP. They are releasing a new tool call Hot Reload
that will make designing Xamarin.Forms app easier because it support native controls 100% (but it's not RTM yet).
If the control is a native control, it won't work in the designer because it requires the native platform to compile and render it via the platform renderer (many of our controls are native controls). Though if the control is a SkiaSharp-based control, then it will sometimes work in the designer assuming all resources are built.
This brings me to the MergedResources part of the XAML, I'm not sure that the designer is properly merging the ResourceDictionaries at design time, thus leads to what you're seeing.
Let me know how it goes after you compile and deploy the project.
Lance | Technical Support Engineer, Principal
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