public Style OperationButtonStyle { get; set; }

Public Property OperationButtonStyle As Style

<Window ...
    xmlns:dxe="http://schemas.devexpress.com/winfx/2008/xaml/editors"
    xmlns:dxet="http://schemas.devexpress.com/winfx/2008/xaml/editors/themekeys">
    <StackPanel>
        <StackPanel.Resources>
            <ResourceDictionary>
                <ResourceDictionary.MergedDictionaries>
                    <ResourceDictionary Source="/DevExpress.Xpf.Themes.Office2019Colorful.v20.2;component/Core/Core/Editors/Themes/Generic/Generic.xaml"/>
                </ResourceDictionary.MergedDictionaries>
            <Style x:Key="digitStyle" TargetType="Button" BasedOn="{StaticResource {dxet:CalculatorThemeKey ResourceKey=ButtonDigitStyle, ThemeName=Office2019Colorful}}">
                <Setter Property="Background" Value="#FFFCFCFC" />
                <Setter Property="Padding" Value="5,15"/>
                <Setter Property="BorderBrush" Value="Transparent" />
                <Setter Property="FontSize" Value="12"/>
                <Setter Property="FontWeight" Value="Bold"/>
                <Style.Triggers>
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter Property="Background" Value="#FFE1E1E1" />
                        <Setter Property="BorderBrush" Value="Transparent" />
                    </Trigger>
                    <Trigger Property="IsPressed" Value="True">
                        <Setter Property="Background" Value="#FFCBCBCB" />
                        <Setter Property="BorderBrush" Value="Transparent" />
                    </Trigger>
                </Style.Triggers>
            </Style>
            <Style x:Key="operationStyle" TargetType="Button" BasedOn="{StaticResource {dxet:CalculatorThemeKey ResourceKey=ButtonOperationStyle, ThemeName=Office2019Colorful}}">
                <Setter Property="Background" Value="#FFF7F7F7" />
                <Setter Property="Padding" Value="5,15"/>
                <Setter Property="BorderBrush" Value="Transparent" />
                <Style.Triggers>
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter Property="Background" Value="#FFE1E1E1" />
                        <Setter Property="BorderBrush" Value="Transparent" />
                    </Trigger>
                    <Trigger Property="IsPressed" Value="True">
                        <Setter Property="Background" Value="#FFCBCBCB" />
                        <Setter Property="BorderBrush" Value="Transparent" />
                    </Trigger>
                </Style.Triggers>
            </Style>
            <Style x:Key="ceStyle" TargetType="Button" BasedOn="{StaticResource {dxet:CalculatorThemeKey ResourceKey=ButtonCEStyle, ThemeName=Office2019Colorful}}">
                <Setter Property="Background" Value="#FFF7F7F7" />
                <Setter Property="Padding" Value="5,15"/>
                <Setter Property="BorderBrush" Value="Transparent" />
                <Style.Triggers>
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter Property="Background" Value="#FFE1E1E1" />
                        <Setter Property="BorderBrush" Value="Transparent" />
                    </Trigger>
                    <Trigger Property="IsPressed" Value="True">
                        <Setter Property="Background" Value="#FFCBCBCB" />
                        <Setter Property="BorderBrush" Value="Transparent" />
                    </Trigger>
                </Style.Triggers>
            </Style>
            </ResourceDictionary>
        </StackPanel.Resources>
        <dxe:Calculator Background="#FFF2F2F2"
                        CornerRadius="10"
                        DigitButtonStyle="{StaticResource digitStyle}"
                        OperationButtonStyle="{StaticResource operationStyle}"
                        CEButtonStyle="{StaticResource ceStyle}" />
    </StackPanel>
</Window>