Hello, 

In order to use the new functionality one should load the custom font using the ThemeResolutionService class and register it in RadRichTextEditor:

ThemeResolutionService.LoadFont(File.OpenRead(@"..\..\Fonts\HelveticaLt.ttf"));
Telerik.WinForms.Documents.Layout.FontManager.RegisterFont(new Telerik.WinControls.RichTextEditor.UI.FontFamily("Helvetica-Light"));

If you are using the RichTextEditorRibbonBar in the application, you need to load and register all custom fonts before initializing the ribbon bar. Here is the code snippet how can achieve it : 

public SampleForm()
{
	ThemeResolutionService.LoadFont(File.OpenRead(@"..\..\Fonts\HelveticaLt.ttf"));
	Telerik.WinForms.Documents.Layout.FontManager.RegisterFont(new Telerik.WinControls.RichTextEditor.UI.FontFamily("Helvetica-Light"));

	InitializeComponent();
	this.richTextEditorRibbonBar1.AssociatedRichTextEditor = this.radRichTextEditor1;

	this.radRichTextEditor1.Document = ImportDocx(file);
}

This improvement will be available in our next release - R1 2018, scheduling for January 2018. 

Best regards, 
WinForms team