Drawing border around marked text in Forms.RichTextbox or Control.RichTextbox

Hello guys!

I have a small but annoying problem concerning RichTextBox. I want to draw a two-color border line around a marked text area (selection) with XAML. The typical border funktion is drawing a border around marked lines and not marked area and it's just one color. It looks like underlined in most feeds - very ugly. I need two colors, because of changing color background of marked text and box.

So I tried artificially measure the distance to in pixels to left box border, width of chars, string etc, but it doesn't work in warping text, because I have no clue how to measure distance, if you resize box. Esp. area is not rectangle (paragraph, end line etc).
Beside, I got feeling that's blooty overkill.

Do you have some hints or ideas about properties, methods or events in .Net 4.0 framework for me dealing with the problem?

Greetings
St. Claus