Editing default message form

[lvr123]

Hello,
I want to edit the default form for reading messages and add a new and editable "comment" field.

I succeeded to edit the form for one particular message but this modification is not available for the other messages. Besides the form for this message now look ugly: the forms shows now with the old all-grey Look-and-Feel.

So my questions are:
- which form must I edit ?
- when edit is done, how do I make this modification available for every messages ?


- how do I keep the original Outlook L&F ?

Kind regards,

Laurent