I used this guide to add the rounded borders: http://www.phpbb-seo.com/en/phpbb-forum/article2427.html (see one of the first posts in the topic)

problem: background image repeats itself after increasing the width of the portal

common.css

/* General proSilver Markup Styles ...

edit:

deleted prev. rounded corners as it is not IE compatible

Your Portal Version: 1.0.5
Your phpBB Type: Standard phpBB3
MODs installed: No
Your knowledge: Beginner
Boardlink: http://www.malawicichlidenforum.nl/phpBB3

What have you done before the problem was there?
I changed the background colour and added the white borders. Then my forum went ...