The Theme preview only shows a default view of dkret3.
If you compare the preview with the themes thumbnail you may miss the bottombars UNDER the content. These bottombars are only shown if at least ONE widget is added to it.
Furthermore you can choose from diffrent scheme files inside the themes admin panel to furthermore change the appearance:
Show dkret3 without any sidebar, have two sidebars on left or right, etc. You can even choose to display one sidebar on top of your content.
I already added all those css styles to the upcomming 2.3 version that prevent the theme preview from looking good (added some missing headings, style for tables etc.)