Hey there Karoline,
Hope you're well today!
If you are referring to the widget bottom border in the sidebar http://screencast.com/t/bXHFouIf. This can be done by adding some custom CSS. The easiest way to do that, if your theme doesn't already have custom CSS tab, would be to add it in your site using the following plugin
http://wordpress.org/plugins/simple-custom-css
Once the plugin is installed and activated you can add this code to Appearance >> Custom CSS:
.widget {
border-bottom: none;
}This should remove the border below the widgets in the sidebar.
Best regards,
Bojan