How to set size mode mini and opacity for application bar on Windows Phone with xamarin.forms

today i was testing ToolbarItems with XF. i was able to add items and menu items.
but I found no way to customize the application bar on WP.

I want to make the appbar very small, using mini size mode.

more details about the options can be found here:

http://msdn.microsoft.com/en-us/library/windows/apps/ff431813(v=vs.105).aspx

is this currently possible with xamarin.forms ?
if not, which way to go ?

Thanks, DanB
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