Hi Jaroslav Půbal,
When you set StyleManager.Theme on a Control in Silverlight the StyleManagers finds a Style in the Theme ResourceDictionaries that can be applied on the Control and sets the Control's Style property. It is not possible to go there and search for a specific Key or otherwise use StaticResource to reference resource in the Themes.
Eventually you can get our theme projects and add styles in them for your custom ContentControl that you can use as Border and the Theme for it will just change the Background and BorderBrush. This will be overkill though if all you need is to set a brush depending on the selected theme.
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