Ok you mean in the VS designer. It looks like Microsoft has added some bugs in the designer that aren't properly finding resources that are used by our designer integration code. For instance, I can sometimes open the solution, make the change to the RequestedTheme, and it wll work fine. And then if I make any other changes this problem starts happening where the resource isn't found. Then I have been able to close the designer, rebuild, and reopen it and have it work. We will contact Microsoft on this to see what they can do to help fix it.
If you do the code in XAML only mode or C# code behind, you shouldn't run into the designer issue and it will execute at run-time.
That being said, SyntaxEditor doesn't currently define a separate dark theme. It will use brushes like TextBoxBackgroundThemeBrush (system ones) that come from the app's theme, meaning it will be dark if your app's theme is Dark. You can alter properties like SyntaxEditor.Background, BorderBrush, etc. to override that. But to affect the text area, you would need to update the highlighting styles that are registered for each classification type used by the syntax languages you harness. By default the highlighting styles we define in our languages are for a light background editor. All of that is easily changed though, especially if you are just using a dark theme since you can change the various colors right in the languages.