You must use the InvokeScriptcontrol's method WebBrowserto introduce some Javascript code, which in turn will change the properties of the element, such as background and foreground colors.
:
public MainPage()
{
InitializeComponent();
this.webBrowser1.IsScriptEnabled = true;
this.webBrowser1.Navigate(new Uri("http://yourpage.com/"));
this.webBrowser1.LoadCompleted += webBrowser1_LoadCompleted;
}
void webBrowser1_LoadCompleted(object sender, System.Windows.Navigation.NavigationEventArgs e)
{
this.webBrowser1.InvokeScript("eval", new[] { "document.body.style.background = 'black';" });
}