Thank you very much for your suggestions Nagel, they are much appreciated! In terms of editing the define_subscribe.php page with <br> I have tried that before without any results.... After playing around with the Web Developer tool kit I also came to the conclusion that it's something to do with the css files of my site and I've tried editing them, but again without any success (I must admit that I'm not very good with css or with programming in general), so if you could give me some tips on how I might add a new class to my css file(s) and also where in that file I would be forever grateful :) If you need me to post the css files (or even to send them to you) in order for you to get a better idea then I can do so as well.
Regarding the .centerColumn I've actually noticed that it is already declared in one of my css files (stylesheet_main.css), so I'm not sure why the newsletter page is not picking it up....