Customize the window alert box.
Replaces the generic window alert box with a styleable modal. The generic alert box often appears on the one page checkout when the credit card number is invalid. This extension lets you style the box as well as configure the header and body text.
Upload files to root of your Magento install. Configure the extension in the admin in System -> Configuration -> General -> Design -> Stylert. Enable the extension and optionally set the header and message after text:
