Every entry in a ResourceDictionary requires a dictionary key. This is not true when we try to define a style. In that case is generated automatically and all the controls of type defined in the TargetType will be applied the custom style.
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="App1.Page1">
<ContentPage.Resources>
<ResourceDictionary>
<Style TargetType="Label">
<Setter Property="HorizontalOptions" Value="Center" />
<Setter Property="VerticalOptions" Value="CenterAndExpand" />
<Setter Property="TextColor" Value="Red" />
<Setter Property="FontSize" Value="Large" />
</Style>
</ResourceDictionary>
</ContentPage.Resources>
<StackLayout>
<Label x:Name="MyLabel" Text="Hello World!" />
</StackLayout>
</ContentPage>