I'm just beginning with the syntax editor, so am using a simple test project with nothing but a RadSyntaxEditor in a MainWindow.
I am using the separate NuGet packages (version 2020.1.115) with implicit styles (Fluent theme).
What I find is that even with nothing else, I get an error from my App.xaml file saying "Cannot find resource named 'DragVisualStyle'. Resource names are case sensitive." This doesn't seem to prevent the project from building and running, but still it is an error that shows in the error list and I seem to be unable to get rid of it.
After a small amount of investigation, it does seem that the resource dictionary for the SyntaxEditor (at least for the Fluent theme, but maybe others) is referencing the DragVisualStyle without defining it. Looking at some of the other controls it seems that they do each redefine that style if they need it, so the SyntaxEditor seems to differ in that respect.
I wanted to attach a simple example solution, but can't attach .zip files, so basically reproducing this amounts to creating a new WPF project, adding PackageReferences to the SyntaxEditor and Fluent theme NuGet packages, then merging the following in App.xaml:
<ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.xaml" />
<ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/System.Windows.xaml" />
<ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.Input.xaml" />
<ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.SyntaxEditor.xaml" />
And having the following in MainWindow.xaml: