We are constantly working on improving the theming support of RadRichTextBox. In Q3 release we better the UI dialogs, however replacing the default large icon sets needs more time, so we postponed it for Q3 SP1. This task now has highest priority for us and we will do our best to complete it by SP1 which should be out in several weeks. The problematic icons however are used in the generated XAML only, thus you can replace them by directly editing your XAML file and pointing buttons images to your custom icons.
Please, excuse us for the inconvenience. Don't hesitate to contact us if you have other questions.
All the best,
the Telerik team
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