I love this plugin! However, I want to do a few modifications to it, and I have not been able to figure it out by messing with the CSS. First, I want to keep the button to the right, but I want it vertically centered on the screen. Second, my button gets hidden by my footer if it reaches down there. How can I make it stay on top? Lastly, how can I make the button disappear if viewed on a mobile device?