How do I change button default background color on mouse over event in wpf?

You need to modify control template. You can refer below example

<Style TargetType="{x:Type Button}">           

            <Setter Property="Background" Value="{StaticResource blackButtonSOWBrush}"/>

            <Setter Property="Foreground" Value="White"/>           

            <Setter Property="Width" Value="100"/>

            <Setter Property="Template">

                <Setter.Value>

                    <ControlTemplate TargetType="{x:Type Button}">

                        <Border Background="{TemplateBinding Background}">

                            <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>

                        </Border>

                    </ControlTemplate>

                </Setter.Value>

            </Setter>

            <Style.Triggers>

                <Trigger Property="IsMouseOver" Value="False">

                    <Setter Property="Background" Value="{StaticResource blackButtonSOWBrush}"/>

                    <Setter Property="Foreground" Value="White"/>

                    <Setter Property="Opacity" Value="1"></Setter>

                </Trigger>

                <Trigger Property="IsMouseOver" Value="True">

                    <Setter Property="Background" Value="{StaticResource blackButtonSOWBrush}"/>

                    <Setter Property="Opacity" Value="0.8"></Setter>

                    <Setter Property="Foreground" Value="Blue"></Setter>

                </Trigger>

              

            </Style.Triggers>

        </Style>