Style page for smaller screens (CSS)

Hi,

I have this signup page for my multisite: http://www.bloggerspoint.dk/opret-blog/ and i want to make it work like this page: http://bloggersdelight.dk/opret-blog/ when it is visited on smaller screens. Right now it almost does it itself, but I need to adjust the width of each row, as on the bloggersdelight site.
Right now both rows stick to each side, and are very wide, and i don't want that, as it look very bad.