Custom error page

Hi guys

I am being told our members don t like to land on the default error page that tells them that they are not logged in. Probably because the default language is confusing for people who partied all night. I was wondering if there was an addon or something I could modify with some custom css or code to have a nice looking message well structured with bullet points telling them what to do instead of the forums error you get when someone tries to get into the client section here.

Let me know if there is something out there or where to change the error page template

Thanks in advance