you need to use Static Resource, I find a good resource for you:
https://blog.xamarin.com/easy-app-theming-with-xamarin-forms-styles/
So you need to do the following:
1- Define ResourceDictionaryat the application level in App.xaml
<Application
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="MonkeyTweet.App">
<Application.Resources>
<ResourceDictionary>
<Color x:Key="backgroundColor">#33302E</Color>
<Color x:Key="textColor">White</Color>
</ResourceDictionary>
</Application.Resources>
</Application>
2- StaticResource :
<Label Text="{Binding Text}" TextColor = "{StaticResource textColor}"/>