this was a permisions error.
on trying to access the CSS in PowerMessenger/webassist/themes/Default/css/default_styles.css
i got the following error:
You don't have permission to access /PowerMessenger/webassist/themes/Default/css/default_styles.css on this server.
using file zilla i changed the permissions for the webassist folder and all files and folders under it to 755 to correct the problem