Nice plugin. I installed it and selected theme 1, then checked the custom CSS box...next I went to plugin editor and copied the theme1 css file and pasted it in the custom CSS box on the settings page....made a modal on a page (using shortcode), clicked it, then used Firebug to inspect it.
A little bit later, I had the modal looking great and matching my site except one little weird issue:
The messages that show up on the top of the login window during authentication and successful login (but NOT when it fails) have a little 4 or 5 pixel wide area on the left edge that is still the original color of the theme. (I changed all the green elements to a different color). But heading looks great, button looks great, mouseovers work, focus colors for the inputs are right...just this last little item I can't figure. I assume it's a div that is styled to have that background color but I can't find it!
Can I get an email to send you the CSS I'm using. My site is still in development so it's got a Private Site plugin....if I can have an email to contact you I can give you the password and the page the modal is on so you can see what it looks like.
On a completely unrelated noted...I wish I could figure out how to get the login / logout links to show up in my WP nav...shortcodes don't work in the menu admin.