Update :
Since the release of Windows Phone 8.1, you can set the RequestedTheme attribute on any control, or even at the app level to override the Theme set by the users in the Settings.
Example to force the Light theme:
In code, within the constructor of the App class :
/// <summary>
/// Provides application-specific behavior to supplement the default Application class.
/// </summary>
public sealed partial class App : Application
{
private TransitionCollection transitions;
/// <summary>
/// Initializes the singleton application object. This is the first line of authored code
/// executed, and as such is the logical equivalent of main() or WinMain().
/// </summary>
public App()
{
this.RequestedTheme = ApplicationTheme.Light;
this.InitializeComponent();
this.Suspending += this.OnSuspending;
}
}
Or in XAML :
<Application
x:Class="App26.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
RequestedTheme="Light"
xmlns:local="using:App26">
</Application>
For the old Windows Phone 8 application model :
It is of course recommended in the design guidelines to use the theme resources to make sure your app works with any theme and accent color.
However if you really want to force the dark theme, here is a solution provided by Rudy Huyn on his blog: http://www.rudyhuyn.com/blog/2013/01/18/forcer-un-theme-sous-windows-phone-8/
The idea is to add a method in your App class that will override all the system brushes with the colors of the dark theme:
private void DarkTheme()
{
((SolidColorBrush)Resources["PhoneRadioCheckBoxCheckBrush"]).Color = ((SolidColorBrush)Resources["PhoneRadioCheckBoxBorderBrush"]).Color = ((SolidColorBrush)Resources["PhoneForegroundBrush"]).Color = Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneBackgroundBrush"]).Color = Color.FromArgb(0xFF, 0x00, 0x00, 0x00);
((SolidColorBrush)Resources["PhoneContrastForegroundBrush"]).Color = Color.FromArgb(0xFF, 0x00, 0x00, 0x00);
((SolidColorBrush)Resources["PhoneContrastBackgroundBrush"]).Color = Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneDisabledBrush"]).Color = Color.FromArgb(0x66, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneProgressBarBackgroundBrush"]).Color = Color.FromArgb(0x19, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneTextCaretBrush"]).Color = Color.FromArgb(0xFF, 0x00, 0x00, 0x00);
((SolidColorBrush)Resources["PhoneTextBoxBrush"]).Color = Color.FromArgb(0xBF, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneTextBoxForegroundBrush"]).Color = Color.FromArgb(0xFF, 0x00, 0x00, 0x00);
((SolidColorBrush)Resources["PhoneTextBoxEditBackgroundBrush"]).Color = Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneTextBoxReadOnlyBrush"]).Color = Color.FromArgb(0x77, 0x00, 0x00, 0x00);
((SolidColorBrush)Resources["PhoneSubtleBrush"]).Color = Color.FromArgb(0x99, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneTextBoxSelectionForegroundBrush"]).Color = Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneButtonBasePressedForegroundBrush"]).Color = Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneTextHighContrastBrush"]).Color = Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneTextMidContrastBrush"]).Color = Color.FromArgb(0x99, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneTextLowContrastBrush"]).Color = Color.FromArgb(0x73, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneSemitransparentBrush"]).Color = Color.FromArgb(0xAA, 0x00, 0x00, 0x00);
((SolidColorBrush)Resources["PhoneChromeBrush"]).Color = Color.FromArgb(0xFF, 0x1F, 0x1F, 0x1F);
((SolidColorBrush)Resources["PhoneInactiveBrush"]).Color = Color.FromArgb(0x33, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneInverseInactiveBrush"]).Color = Color.FromArgb(0xFF, 0xCC, 0xCC, 0xCC);
((SolidColorBrush)Resources["PhoneInverseBackgroundBrush"]).Color = Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFF);
((SolidColorBrush)Resources["PhoneBorderBrush"]).Color = Color.FromArgb(0xBF, 0xFF, 0xFF, 0xFF);
}
Then, in the App constructor, you check whether the Light theme is enabled, and if it is, you override the theme:
if ((Visibility) Resources["PhoneLightThemeVisibility"] == Visibility.Visible)
{
DarkTheme();
}