set margin width locally doesn't work

[najja]

hi there,

I like having two blank margin flanking the text when I do my daily
editing so I add this to my .emacs:

(setq default-left-margin-width 20)
(setq default-right-margin-width 20)

However, now I want to extend my emacs usage to other fields such as
email and rss reading. However, this margin setting will create a
blank margin in gnus and newsticker buffer just like in others. I have
tried to add the following settings to .emacs.

(add-hook 'newsticker-treeview-list-mode-hook
             (setq left-margin-width 0)
             (setq right-margin-width 0)))

but it doesn't help. Obviously setting margin width doesn't take
effect immediately. Is there any way to set margin width to a buffer
locally and effectively?