I just want to make some short notes about themes in Xamarin.Forms Apps, since there are already a lot of tutorials available online, that go way deeper.

Themes
Themes are actually ResourceDictionaries. Each atomic type you use in your XAMLs and Styles, will be defined here. Types are f.e. Color, Double (for font sizes), etc.

<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
                    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                    x:Class="ShoppingList.Themes.DarkTheme">
    <Color x:Key="BackgroundColor">#333333</Color>
    <Color x:Key="TextColor">#eeeeee</Color>
    <Color x:Key="PlaceholderTextColor">#55ffffff</Color>
    <Color x:Key="ControlBackground">#111111</Color>
    <Color x:Key="ButtonBackgroundColor">#11ffffff</Color>
</ResourceDictionary>

Be sure to have a code behind file for your ResourceDictionary, that calls InitializeComponent(). Otherwise this will only work on UWP (without .NET Toolchain) and no other platforms.

Styles
In most cases the styles are not part of your themes, because using themes just makes you change the look of your pages and controls, not the styling of specific controls. Perhaps styles are using the resources, that you define in your theme. Be sure to use DynamicResource for linking to the key of the resource, otherwise you can not change themes during runtime. (Restart will be needed)

<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
                    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                    x:Class="ShoppingList.Resources.Styles">
    <Style TargetType="NavigationPage">
            <Setter Property="BackgroundColor" Value="{DynamicResource BackgroundColor}"/>
            <Setter Property="BarBackgroundColor" Value="{DynamicResource BackgroundColor}"/>
            <Setter Property="BarTextColor" Value="{DynamicResource TextColor}"/>
            
        </Style>
        <Style TargetType="Label">
            <Setter Property="TextColor" Value="{DynamicResource TextColor}" />
        </Style>
        <Style TargetType="Entry">
            <Setter Property="BackgroundColor" Value="{DynamicResource ControlBackground}" />
            <Setter Property="TextColor" Value="{DynamicResource TextColor}" />
            <Setter Property="PlaceholderColor" Value="{DynamicResource PlaceholderTextColor}" />
        </Style>
        <Style TargetType="Button">
            <Setter Property="TextColor" Value="{DynamicResource TextColor}" />
            <Setter Property="BackgroundColor" Value="{DynamicResource ButtonBackgroundColor}" />
            
        </Style>
</ResourceDictionary>

Changing the theme
Now you just need to add the following code to your App.xam.cs, then you can just call App.SetTheme(typeof(DarkTheme)) to change the theme during runtime.

Be sure to add all your style-ResourceDictionaries and a standard theme to your App.xaml in the Application.Resources-Section

internal static void SetTheme(Type themeType)
{
   Preferences.Set("Theme", themeType.FullName);
   var resDict = (ResourceDictionary)Activator.CreateInstance(themeType);
   App.Current.Resources.MergedDictionaries.Add(resDict); // this line replaces all keys of the current dictionary with the value of the selected dictionary, only the keys that are present in the selected dictionary will be replaced
}

In your “OnInitialize” method of your App.xaml.cs you can put the following code, to load the saved theme on startup:

if (Preferences.ContainsKey("Theme"))
{
   var themeTypeName = Preferences.Get("Theme",null);
   var themeType = Type.GetType(themeTypeName);
   if (themeType != null)
   {
      SetTheme(themeType);
   }
}