Force Windows Phone Theme

Windows Phone applications inherits the theme of the OS on launch.

When the UI is designed specially for the dark theme it won't look well on the light theme, or vice versa.

To prevent this the application can force the default dark or light theme.

In the application class' constructor put this code to force the dark theme:

if ((Visibility)Application.Current.Resources["PhoneLightThemeVisibility"] == Visibility.Visible) MergeCustomColors("/Themes/DarkStyles.xaml");

Or this code to force the light theme:

if ((Visibility)Application.Current.Resources["PhoneDarkThemeVisibility"] == Visibility.Visible) MergeCustomColors("/Themes/LightStyles.xaml");

Anywhere in your project put this method:

private void MergeCustomColors(String Theme)
{
    ResourceDictionary Dictionaries = new ResourceDictionary();
    String source = String.Format(Theme);
    var themeStyles = new ResourceDictionary { Source = new Uri(source, UriKind.Relative) };
    Dictionaries.MergedDictionaries.Add(themeStyles);
    ResourceDictionary appResources = Current.Resources;
    foreach (DictionaryEntry entry in Dictionaries.MergedDictionaries[0])
    {
        SolidColorBrush ColorBrush = entry.Value as SolidColorBrush;
        SolidColorBrush ExistingBrush = appResources[entry.Key] as SolidColorBrush;
        if (ExistingBrush != null && ColorBrush != null) { ExistingBrush.Color = ColorBrush.Color; }
    }
}

The code assumes that the projects contains the files DarkStyles.xaml and LightStyles.xaml in a folder named Themes.

A demo project can be found here.