<TextBox>
    <TextBox.Style>
        <Style TargetType="TextBox">
                <Setter Property="FontWeight" Value="Bold" />
                <Style.Triggers>
                <Trigger Property="Text" Value="default">
                    <Setter Property="FontWeight" Value="Normal" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>