Hi Stergios,
You can use the active editor presenter to determine the document position from a point obtained in a mouse event the following way:
Such event can be the MouseMove of the RadRichTextBox. For performance reasons you can limit getting the position to several times in a second.
Having the section you need, you can apply any style.
Don't hesitate to contact us if you have other questions.
All the best,
Martin
the Telerik team