Update the Terms of Service button color when creating a WordPress.com account

I am not totally sure this is the right way to attack this keeping in mind that the string needs to be translatable.
cc @astralbodies 
Fixes #1645

![ios simulator screen shot sep 19 2014 5 03 02 pm](https://cloud.githubusercontent.com/assets/363753/4343218/63afd702-4051-11e4-9b26-29cfb361f3f4.png)