TextBox with background in WP7 Silverlight

I have a textbox on which i set the backgrou开发者_运维技巧nd property. This is valid if I don't change the background theme in WP7 settings. When I change this, the property is overriden and the control does not have the correct background anymore. How can I make this background the same for both themes?

<TextBox Canvas.Left="12" Canvas.Top="482" Height="71" Name="someTextBox" Text="Some text..." Width="456" GotFocus="SomeTextBox_GotFocus" Background="White" Foreground="Black" BorderBrush="White" />