Change background color of ToolbarItems

How do I change the background color and text of ToolbarItems? The default color is black and I would like to change how to do it?

<ContentPage.ToolbarItems>
    <ToolbarItem Order="Secondary" Icon="morevert.png" Text="Atualizar" Command="{Binding Atualizar}"/>
    <ToolbarItem Order="Secondary" Text="Ajuda" Command="{Binding Ajuda}"/>
    <ToolbarItem Order="Secondary" Text="Sair" Command="{Binding Sair}"/>
</ContentPage.ToolbarItems>