.CSS HTML Help

Hello,

I'm stuck on configuring a .CSS file for custom branding of a MFT site. We are using Serv-U MFT running on Ubuntu 16.04.2 LTS. I've somehow managed to get the background correct on the login page, but the Login/Password fields are too high- top center. I'm not sure of how to move it down to the center of the page. I have attached a screenshot of what it looks like.

Here's the code from the .css file I have made:

html {
      background: url('/%25CUSTOM_HTML_DIR%25/Images/bg.jpg');
      -webkit-background-size: cover;
      -moz-background-size: cover;
      -o-background-size: cover;
      background-size: cover;

}