Thunderbird: Trying to add a checkbox to "Feed properties" window

[Shay Morag]

I'm new to extension developing, so forgive my obvious ignorant questions.
I want to mark certain feeds to open as RTL (right to left).

So I decided I'll add a check-box to the Feed Properties window.
The problem is I can't seem to be able to customize it.
I found it in the \chrome\newsblog.jar, which has the following text:
...content/messenger-newsblog/feed-properties.xul... (without the ... of
course)

So I wrote the following to test if it displays:

******* chrome.manifest *************************
overlay
chrome://messenger-newsblog/content/messenger-newsblog/feed-properties.xul
chrome://myfirstext/content/myhelloworld.xul
************************************************

******** myhelloworld.xul ************************
<?xml version="1.0"?>
<overlay id="test"
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<dialog id="feedPropertyDialog"
xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<label value="monkeymonkey" accesskey="&feedLocation.accesskey;"
control="monkeymonkey"/>
</dialog>
</overlay>
******************************

Of course it didn't.

If anyone can help I will be very thankful.

P.S. If you also know how to show a message as RTL it will also be
greatly appreciated.

Thanks in advance