<Window.Resources>
    <Style TargetType="Button">
        <Setter Property="Background" Value="Blue" />
        <Style.Triggers>
            <Trigger Property="IsMouseOver" Value="True">
                <Setter Property="FontWeight" Value="Bold" />
            </Trigger>
        </Style.Triggers>
    </Style>
    <Style x:Key="style1">
        <Setter Property="TextBlock.Padding" Value="20" />
    </Style>
</Window.Resources>
<StackPanel>
    <Button>This is a button</Button>
    <TextBlock Style="{StaticResource style1}">This is a text block</TextBlock>
</StackPanel>

WPF: Styles & Triggers

C#