Hi mkhalil77,
this is accomplished by creating a theme. I’d suggest to use the example theme https://github.com/owncloud/theme-example (direct link to the zip: https://github.com/owncloud/theme-example/archive/master.zip).
There you can remove the core folder, instead create the directory /apps/activity/templates and copy the files email.notification.php and html.notification.php from the activity app.
Make your customization, copy your theme to your apps directory and enable it. Make sure the theme directory is readable by your www user.
Also, there are some limits with the theme identifier, see this post: Activate a theme