Where are you loading your CSS files?
You may find that your CSS files are loaded somewhere which doesnt cover security pages.
Have you created a template .ss file for your Security page?
You may need a template created in order for CSS to apply.
You can simply create a "Security.ss" file inside "/theme/YOURTHEME/templates/Layout".
The css is in the theme folder theme/paratus/css and on http://www.kundregistrering.se/Security/login?BackURL=%2Fadmin%2Fpages if i check the code in firebug the css path is correct but when i expand the css in firebug the whole html is there but as you mentioned, i will creat a security.ss file in the layout, what should it contain? It was a while since i worked on Silverstripe.