OK, Here is the problem. In prosilver theme there is no text like this in the posting_body.html file:

<textarea name="message" rows="15" cols="76" tabindex="3" onselect="storeCaret(this);" onclick="storeCaret(this);"
onkeyup="storeCaret(this);" style="width: 98%;">{MESSAGE}</textarea>

Instead of it I have:

<!-- IF S_PRIVMSGS -->
	<!-- INCLUDE posting_pm_layout.html -->
<!-- ELSE -->
	<!-- INCLUDE posting_layout.html -->
<!-- ENDIF -->

And, of course, in posting_pm_layout.html and posting_layout.html there is no such text too.

So doing all you wrote with subsilver2 theme I have an exellent result. But what can I do with prosilver?

P.S. another problem is that TWO EquationEditor windows opens. (On your forum http://tinyurl.com/ylq4pqd there are also two popup windows when you click on Equation Editor button).