Shaker/Avalonia/SukiUI/Theme/Calendar/Calendar.axaml

<ResourceDictionary xmlns="https://github.com/avaloniaui" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ControlTheme x:Key="SukiCalendarStyle" TargetType="Calendar">
        <Setter Property="Background" Value="Transparent" />
        <Setter Property="BorderThickness" Value="0" />
        <Setter Property="HeaderBackground" Value="Transparent" />
        <Setter Property="Template">
            <ControlTemplate>
                <Panel Name="PART_Root" ClipToBounds="True">
                    <CalendarItem Name="PART_CalendarItem"
                                  Background="{TemplateBinding Background}"
                                  BorderBrush="{TemplateBinding BorderBrush}"
                                  BorderThickness="{TemplateBinding BorderThickness}"
                                  HeaderBackground="{TemplateBinding HeaderBackground}" />
                </Panel>
            </ControlTemplate>
        </Setter>
    </ControlTheme>
    <ControlTheme x:Key="{x:Type Calendar}"
                  BasedOn="{StaticResource SukiCalendarStyle}"
                  TargetType="Calendar" />
</ResourceDictionary>