Add Classic DataGrid theme to App.axaml

Author: Anixx

src/UniGetUI.Avalonia/App.axaml

@@ -21,6 +21,7 @@
         <Style Selector="Window">
             <Setter Property="Theme" Value="{StaticResource ClassicWindow}" />
         </Style>
+        <StyleInclude Source="avares://Classic.Avalonia.Theme.DataGrid/Classic.axaml"/>
         <StyleInclude Source="avares://AvaloniaEdit/Themes/Fluent/AvaloniaEdit.xaml"/>
         <StyleInclude Source="avares://UniGetUI/Assets/Styles/Styles.Common.axaml"/>
     </Application.Styles>