<Window.Resources>
<Style TargetType="Button">
<Setter Property="Foreground" Value="#fff"/>
<Setter Property="Background" Value="#FF0F4FC5"/>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" Value="#FF00091B" />
</Trigger>
</Style.Triggers>
</Style>
</Window.Resources>
<Window.Resources>
<Button Content="Test" Height="100" Margin="18,125,51,138" />
<Style x:Key="MyStyle" TargetType="Button">
...
<Button Content="Test" Height="100" Margin="18,125,51,138" Style="{StaticResource MyStyle}" />