The question is not so much because of practical necessity, but because of the desire to understand the intricacies.
In Win 7 it was possible to override keys used in theme item templates.
As far as I understand, this was due to the fact that such templates were possible in the topics:
<SolidColorBrush x:Key="SomeControl.Background">Red</SolidColorBrush>
<Style TargetType="{x:Type ns:SomeControl}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ns:SomeControl}">
<Border Background="{DynamicResource SomeControl.Background}"/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
In Win 10, DynamicResource will no longer be able to find the resource specified in the template. Therefore it is necessary to set StaticResource.
Because of this, the solutions used in Win 7 will not work in Win 10.
<ns:SomeControl>
<ns:SomeControl.Resources>
<SolidColorBrush x:Key="SomeControl.Background">Green</SolidColorBrush>
</ns:SomeControl.Resources>
</ns:SomeControl>
I'm wondering what caused it, what changed in the system, that it stopped working?
Supplementing the question in connection with @BionicCode comment.
XAML of an example using DynamicResource in a theme template:
<UniformGrid Columns="1" >
<ns:SomeControl/>
<ns:SomeControl>
<ns:SomeControl.Resources>
<SolidColorBrush x:Key="SomeControl.Background">Green</SolidColorBrush>
</ns:SomeControl.Resources>
</ns:SomeControl>
</UniformGrid>
The example runs on OS Win 10 and Visual Studio 2022 with all the latest updates at the moment.
Window after launch:
At the moment, I do not have a computer with Win7 available, so I cannot clearly demonstrate the execution of the same XAML.
But if someone runs the same example on Win 7, they will get this result:
