Question

How to register DependencyProperty on Rectangle fill, so i can change the color
dynamically?

<UserControl.Resources>
    <Style x:Key="ButtonStyle1" TargetType="{x:Type Button}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type Button}">
                    <Grid>
                        <Rectangle Stroke="Black">
                            <Rectangle.Fill>
                                <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                                    <GradientStop Color="#FF48B6E4" Offset="0.013"/>
                                    <GradientStop Color="#FF091D8D" Offset="1"/>
                                </LinearGradientBrush>
                            </Rectangle.Fill>
                        </Rectangle>
                        <ContentPresenter HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsFocused" Value="True"/>
                        <Trigger Property="IsDefaulted" Value="True"/>
                        <Trigger Property="IsMouseOver" Value="True"/>
                        <Trigger Property="IsPressed" Value="True"/>
                        <Trigger Property="IsEnabled" Value="False"/>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</UserControl.Resources>

<Grid x:Name="LayoutRoot">
    <Button Style="{DynamicResource ButtonStyle1}"/>
    <TextBlock
        x:Name="NodeName"
        x:FieldModifier="public"
        Text="Property"
        Margin="8"
        HorizontalAlignment="Center"
        VerticalAlignment="Center"
        TextWrapping="Wrap" 
        TextAlignment="Center"
        FontFamily="Segoe Print"
        FontWeight="Bold" 
        Foreground="White"
        FontSize="40"/>
</Grid>

Answer 1

Why don’t you bind Fill to the Background property of the Button:

<ControlTemplate TargetType="{x:Type Button}"> 
    <Grid> 
        <Rectangle Stroke="Black" Fill="{TemplateBinding Background}" /> 
        ...
    </Grid> 
    ...
</ControlTemplate> 

and then set Background like this:

<Button Style="{DynamicResource ButtonStyle1}">
    <Button.Background>
        <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">   
            <GradientStop Color="#FF48B6E4" Offset="0.013"/>   
            <GradientStop Color="#FF091D8D" Offset="1"/>   
        </LinearGradientBrush>   
    </Button.Background>
</Button>