[Enhancement idea] Add ability for developers to override Foreground, FontSize at root of XAML

[giovanni-a]

Currently, if the user sets Foreground at the root of the XAML:

`

<Grid Background="{DynamicResource Theme_BackgroundBrush}">
    <TextBlock Text="Hello, World!" FontSize="23" Margin="32,32,0,0" HorizontalAlignment="Left" VerticalAlignment="Top"/>
    <Button Content="Button" Margin="32,80,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Width="88" Height="32" />
    <RadioButton Content="RadioButton" Margin="32,128,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Width="120" Height="32" />
    <CheckBox Content="Checkbox" Margin="32,176,0,0" HorizontalAlignment="Left" VerticalAlignment="Top" Width="120" Height="32" />
</Grid>

`

it won't affect the text color of Button, RadioButton, CheckBox etc:

This is due to the fact that, in the modern theme, we set Foreground, FontSize, etc. in a style, like this:
<Setter Property="Foreground" Value="{DynamicResource Theme_TextBrush}" />
and the style takes precendence on inherited values.

To solve this problem, I propose that we remove such style setters and, instead, we implement a system that, when a theme is applied, automatically sets Foreground, FontSize, etc. to the RootVisual, so that the property is inherited to the full visual tree, and developers can easily override those properties on a subtree.