Shaker/Avalonia/SukiUI/Theme/ComboBoxStyles.xaml

<Styles
    xmlns="https://github.com/avaloniaui"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:glassMorphism="clr-namespace:SukiUI.Controls.GlassMorphism"
    xmlns:suki="https://github.com/kikipoulet/SukiUI"
    xmlns:theme="clr-namespace:SukiUI.Theme">

    <Design.PreviewWith>
        <Border Padding="20" Background="Transparent">
            <StackPanel
                Background="AliceBlue"
                Orientation="Horizontal"
                Spacing="10">
                <TextBox Text="TEst" />
                <ComboBox Margin="0,0,50,0" PlaceholderText="Select an item">
                    <ComboBoxItem>Item 1</ComboBoxItem>
                    <ComboBoxItem>Item 2</ComboBoxItem>
                </ComboBox>
                <ComboBox
                    Width="200"
                    HorizontalContentAlignment="Center"
                    IsEnabled="False"
                    SelectedIndex="1">
                    <ComboBoxItem>Item 1</ComboBoxItem>
                    <ComboBoxItem>Item 2</ComboBoxItem>
                </ComboBox>
            </StackPanel>
        </Border>
    </Design.PreviewWith>

    <Style Selector="ComboBox">
        <Setter Property="BorderThickness" Value="1,1,1,1" />
        <Setter Property="BorderBrush" Value="{DynamicResource SukiControlBorderBrush}" />
        <Setter Property="CornerRadius" Value="{DynamicResource SmallCornerRadius}" />
        <Setter Property="Background" Value="{DynamicResource SukiBackground}" />
        <Setter Property="HorizontalContentAlignment" Value="Stretch" />
        <Setter Property="VerticalContentAlignment" Value="Center" />
        <Setter Property="Padding" Value="6,6,-6,6" />
        <Setter Property="MinHeight" Value="45" />
        <Setter Property="PlaceholderForeground" Value="{DynamicResource SukiLowText}" />
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Disabled" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border Margin="0,0,0,0" Padding="5">
                    <Border.Styles>
                        <Style Selector="ContentControl.ghost TextBlock">
                            <Setter Property="Foreground" Value="Transparent" />
                        </Style>
                        <Style Selector="ToggleButton:pointerover">
                            <Setter Property="Background" Value="Transparent" />
                        </Style>
                        <Style Selector="ToggleButton:checked">
                            <Setter Property="Background" Value="Transparent" />
                        </Style>
                    </Border.Styles>
                    <suki:GlassCard
                        Name="border"
                        Padding="{TemplateBinding Padding}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}"
                        Classes="Discrete"
                        CornerRadius="{TemplateBinding CornerRadius}">
                        <suki:GlassCard.Transitions>
                            <Transitions>
                                <BrushTransition Property="BorderBrush" Duration="0:0:0.2" />
                            </Transitions>
                        </suki:GlassCard.Transitions>
                        <Grid ColumnDefinitions="*,Auto">

                            <Grid.Resources>
                                <theme:BiggestItemConverter x:Key="BigItem" />
                            </Grid.Resources>
                            <TextBlock
                                Name="PlaceholderTextBlock"
                                Grid.Column="0"
                                Margin="7,1,0,0"
                                HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                VerticalAlignment="Center"
                                Foreground="{TemplateBinding PlaceholderForeground}"
                                IsVisible="{TemplateBinding SelectionBoxItem,
                                                            Converter={x:Static ObjectConverters.IsNull}}"
                                Text="{TemplateBinding PlaceholderText}" />
                            <ContentControl
                                Margin="7,1,0,0"
                                HorizontalContentAlignment="Left"
                                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                                Classes="ghost"
                                Content="{TemplateBinding ItemsSource,
                                                          Converter={StaticResource BigItem}}"
                                ContentTemplate="{TemplateBinding ItemTemplate}"
                                Foreground="Transparent" />
                            <ContentControl
                                Margin="7,1,0,0"
                                HorizontalContentAlignment="Left"
                                VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                                Content="{TemplateBinding SelectionBoxItem}"
                                ContentTemplate="{TemplateBinding ItemTemplate}"
                                Foreground="{DynamicResource SukiText}" />
                            <ToggleButton
                                Name="toggle"
                                Grid.Column="1"
                                Margin="10,0,0,0"
                                Background="Transparent"
                                BorderThickness="0"
                                ClickMode="Press"
                                CornerRadius="{DynamicResource SmallCornerRadius}"
                                Focusable="False"
                                IsChecked="{TemplateBinding IsDropDownOpen,
                                                            Mode=TwoWay}">
                                <Path
                                    Width="8"
                                    Height="4"
                                    HorizontalAlignment="Center"
                                    VerticalAlignment="Center"
                                    Data="F1 M 301.14,-189.041L 311.57,-189.041L 306.355,-182.942L 301.14,-189.041 Z"
                                    Fill="{DynamicResource SukiText}"
                                    Stretch="Uniform" />
                            </ToggleButton>
                            <Popup
                                Name="PART_Popup"
                                MinWidth="{Binding Bounds.Width, RelativeSource={RelativeSource TemplatedParent}}"
                                MaxHeight="{TemplateBinding MaxDropDownHeight}"
                                HorizontalOffset="-5"
                                IsLightDismissEnabled="True"
                                IsOpen="{TemplateBinding IsDropDownOpen,
                                                         Mode=TwoWay}"
                                PlacementTarget="{TemplateBinding}">
                                <LayoutTransformControl Name="PART_LayoutTransform" RenderTransformOrigin="50%, 0%">

                                    <Border Name="B1" Padding="5,10,5,5">
                                        <Panel>
                                            <Border
                                                Margin="10,-10,0,0"
                                                Background="{DynamicResource SukiCardBackground}"
                                                BorderBrush="{DynamicResource SukiMenuBorderBrush}"
                                                BorderThickness="1,1,1,1"
                                                BoxShadow="{DynamicResource SukiSmallPopupShadow}"
                                                CornerRadius="{DynamicResource SmallCornerRadius}">
                                                <Panel Background="{DynamicResource PopupGradientBrush}">

                                                    <ScrollViewer HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}">

                                                        <ItemsPresenter
                                                            Name="PART_ItemsPresenter"
                                                            Margin="0,3,0,0"
                                                            ItemsPanel="{TemplateBinding ItemsPanel}" />
                                                    </ScrollViewer>
                                                </Panel>
                                            </Border>
                                        </Panel>
                                    </Border>
                                </LayoutTransformControl>
                            </Popup>
                        </Grid>
                    </suki:GlassCard>
                </Border>
            </ControlTemplate>
        </Setter>
    </Style>

    <Style Selector="ComboBox[IsDropDownOpen=true] /template/ LayoutTransformControl#PART_LayoutTransform">
        <Style.Resources>
            <theme:PlusNineConverter x:Key="NineConverter" />
        </Style.Resources>
        <Style.Animations>
            <Animation
                Easing="SineEaseOut"
                FillMode="Forward"
                Duration="0:0:0.3">
                <KeyFrame Cue="0%">
                    <Setter Property="ScaleTransform.ScaleY" Value="0.92" />
                    <Setter Property="ScaleTransform.ScaleX" Value="0.95" />
                    <Setter Property="Opacity" Value="0" />
                </KeyFrame>
                <KeyFrame Cue="100%">
                    <Setter Property="ScaleTransform.ScaleY" Value="1" />
                    <Setter Property="ScaleTransform.ScaleX" Value="1" />
                    <Setter Property="Opacity" Value="1" />
                </KeyFrame>
            </Animation>
        </Style.Animations>
    </Style>

    <Style Selector="ComboBox[IsDropDownOpen=false] /template/ LayoutTransformControl#PART_LayoutTransform">
        <Style.Resources>
            <theme:PlusNineConverter x:Key="NineConverter" />
        </Style.Resources>
        <Style.Animations>
            <Animation
                Easing="CircularEaseOut"
                FillMode="Forward"
                Duration="0:0:0.3">
                <KeyFrame Cue="0%">
                    <Setter Property="ScaleTransform.ScaleY" Value="1" />
                    <Setter Property="ScaleTransform.ScaleX" Value="1" />
                    <Setter Property="Opacity" Value="1" />
                </KeyFrame>
                <KeyFrame Cue="100%">
                    <Setter Property="ScaleTransform.ScaleY" Value="0.9" />
                    <Setter Property="ScaleTransform.ScaleX" Value="0.9" />
                    <Setter Property="Opacity" Value="0" />
                </KeyFrame>
            </Animation>
        </Style.Animations>
    </Style>

    <Style Selector="ComboBox:pointerover /template/ suki|GlassCard#border">
        <Setter Property="BorderBrush" Value="{DynamicResource SukiLightBorderBrush}" />
    </Style>
    <Style Selector="ComboBox:disabled /template/ suki|GlassCard#border">
        <Setter Property="Opacity" Value="{DynamicResource ThemeDisabledOpacity}" />
    </Style>

</Styles>