We have a known issue with this and this is why you cannot set the theme. The issue is logged on our feedback portal. You can track its progress, subscribe to status changes and add your comment to it here: PdfViewer: Implement individual setting of the theme of the RadPdfViewerToolBar with the StyleManager.
Currently, the possible solution is to set the theme of the whole application:
StyleManager.ApplicationTheme = new FluentTheme();
Thank you for your understanding.