The path to your CSS file looks… odd. It needs to be relative to your site root, not to your FTP account — unless there is a folder called "bfthemes.info" inside your web directory (which there doesn't appear to be), you should just be linking to it as "/style.css".
Also, you cannot (and do not need to) style the "html" element in CSS, as it is never rendered on screen. The highest-level element "visible" to CSS is the "body" element. Additionally, if you are going to change the background color, you should probably also set the foreground color (using the "color" attribute), as the user's default colors may clash with the background you set.