For some reason, the modal popup for login is not nicely rounded on the top right and bottom right corners and is even showing a scrollbar (which one can't use).
It' s probably pretty straightforward why it's doing this, but with my (limited) knowledge, I can't see the cause.
I followed your instructions (option 2) and then added the piece of css to the /templates/<MY_JOOMLA_TEMPLATE>/html/mod_sclogin/themes/ folder and more specific the file default.css which I copied from /media/sourcecoast/themes/sclogin/
However, this doesn't seem to change anything.
(btw, I also cleared the browsers cache first)
I'm seeing rounded corners and no scroll bar when I open the popup on your site:
I'm also able to see the CSS change Melissa provided above in the following file, which is included on the page:
/CMS30/templates/psvbcbrugge_3/html/mod_sclogin/themes/default.css
Can you double check, from other browsers and clearing their caches as well, to ensure you're seeing what we see?