how to make the top nav bar invisible when using shell?

I'm trying to use shell , but I want to customize y top nav bar , I tried to find something to change the bachground color and the title color but I didn't find anything, so I thought I can get the navigation bar invisible and do my own but the normal method woudn't work because I think we are not using NavigationPage.
does any one has an idea about this.
thnx.