That is coming from whatever you are using to create the fancy WP login/registration form. It is basically changing all the forms on your site so it is also affecting the login form box in Simple:Press when it opens up and probably elsewhere.
There is an entry in it’s CSS for “.login form, form”
A couple of ways around this you could try.
It probably only needs to be “.login form” you could try changing that or find the entry and remove the box-shadow entry.
Or… You could edit your Simple:Press theme and add
#spMainContainer form {
box-shadow: none;
}
If you change your SP theme You should have your own custom one if you don’t already