title: HTML background image offset by x pixels from the center
body: <p>I'd like to put an image as the background of a webpage but have it offset by some number of pixels with respect to the center.</p>
  
  <p>How can I do this?</p>
  
  <p>I want:</p>
  
  <pre><code>background-image: url("bg.png");
  background-position: 25% center;
  background-attachment: fixed;
  background-repeat: no-repeat;
  </code></pre>
  
  <p>but instead of 25%, I want something along the lines of "center - 50px". Is there any solution to this?</p>