Shaker/Avalonia/SukiUI/Theme/RichTextBoxStyles.axaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:suki="clr-namespace:SukiUI.Controls">
    <Design.PreviewWith>
        <Border Padding="20" Background="{DynamicResource SukiBackground}">
            <StackPanel>
                <!-- <suki:RichTextBox Prefix="Test :" Text="test" /> -->
            </StackPanel>
        </Border>
    </Design.PreviewWith>
</Styles>