Javascript Logic that Changes Optin Form Color

Hey guys -- got a general question I'm hoping some brilliant dev might be able to help me with before I go to tweaky.

I need to add some kind of javascript

What kind of javascript code (guessing a function) would I need to add, in order to change the background and text of an optin form when a vistor scrolls to the bottom of the post?

Example: http://www.shopify.com/blog/7365564-32-key-performance-indicators-kpis-for-ecommerce#axzz2PIo6oN8z

My site:
http://wearabl.com/how-to-price-your-products/

Changes I'd like:

Change background from white to black
Change text from black to white

If you've got time and want to earn some points, I'd really appreciate it :slight_smile:

Cheers