Hi Christian,

It appears that this is a typical feature of the implicit theming mechanism of the .NET Framework. If you have already addressed it at the Microsoft's GitHub repo and they take it into consideration, we will be able to update our mechanism respectively.

Until then, I believe the only suggestion is to switch to StyleManager if the performance using this theming mechanism is satisfactory for you.
If there is anything else I can assist you with, do not hesitate to contact me.

Best Regards,
Vicky
Progress Telerik

Hi Christian,

One of our main goals when it comes to the styling mechanisms we provide is to ensure the consistent look and feel for all Telerik controls. That is the reason why we support styles for the MS controls (referred by you as "standard controls") which are internally used as part of the default ControlTemplates of the more complex Telerik controls.

The above-said excludes the possibility of removing the System.Windows.xaml file from our theme dictionaries.

I am not sure of your exact scenario. I will need more information about it. If we take the example given about the TabbedWindow control:

1.  Do you want it to contain an MS Button which has the MS Style applied?
2. Or do you want all MS controls used in its ControlTemplate to have the MS Styles?

If the answer to the first question is yes, then you have two options depending on the theming mechanism that you prefer to use.

• Option 1 - Use StyleManager. For this Telerik theming mechanism all standalone native controls that we support theme-aware styling for will not automatically receive it. By standalone native controls I mean those which are not part of our controls' default ControlTemplates. For them, as stated in the Use StyleManager to Apply Theme on MS Controls article, you should manually set the corresponding theme.
• Option 2 - Use NoXaml without merging the System.Windows.xaml file in your Application Resources. There is one more thing that needs to be done here - overriding the styles for all native controls supported by the Telerik theming mechanism. Of course, all native controls which you will use as a standalone ones and that you do not want to have Telerik styles. This is recommended to be done in a separate ResourceDictionary that needs to be merged in the Application Resources:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Style TargetType="{x:Type Button}"/>
    <Style TargetType="{x:Type ScrollViewer}"/>
    <Style TargetType="{x:Type CheckBox}"/>
    <Style TargetType="{x:Type TextBox}"/>
    <Style TargetType="{x:Type RadioButton}"/>
    <Style TargetType="{x:Type ListBox}"/>
    <Style TargetType="{x:Type PasswordBox}"/>
    <Style TargetType="{x:Type RepeatButton}"/>
    <Style TargetType="{x:Type ToolTip}"/>
    <Style TargetType="{x:Type Hyperlink}"/>
    <Style TargetType="{x:Type StatusBar}"/>
    <Style TargetType="{x:Type GridSplitter}"/>
    <Style TargetType="{x:Type Separator}"/>
</ResourceDictionary>

<!--And then in your App.xaml-->

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <!--<ResourceDictionary Source="/Telerik.Windows.Themes.Office2019;component/Themes/System.Windows.xaml"/>-->
            <ResourceDictionary Source="/Telerik.Windows.Themes.Office2019;component/Themes/Telerik.Windows.Controls.xaml"/>
            <ResourceDictionary Source="/Telerik.Windows.Themes.Office2019;component/Themes/Telerik.Windows.Controls.Input.xaml"/>
            <ResourceDictionary Source="/Telerik.Windows.Themes.Office2019;component/Themes/Telerik.Windows.Controls.Navigation.xaml"/>
            <ResourceDictionary Source="/NativeControlsStyles.xaml"/>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

I hope you find the above information helpful.

Best Regards,
Vicky
Progress Telerik