The styling is specific to your page. Looking at
www.deherba.com/create-an-account.html there are a couple of things that you need to fix:
* Your "section class="registration" has a width defined of 500px. This needs to be at 100% width.
* Your legend within the form needs to be at 100% width
* Your "#member-registration dt" have a min-width set of 170px, but needs a width 100% too.
* Add a border-bottom to your legend to get the underline.
With these four updates, you will get closer to the styles of our page.
-Melissa