How do I make header bar opaque

How do I make header bar opaque

I have a page with a header bar that contains the HTML title and a Home button. On a long page that scrolls, the header stays in the same place and the text scrolls under it, which is what I want. But the header is transparent and the text is visible under the header as it scrolls past. I want an opaque header, what do I need to change in CSS?

I'm using JQM 1.4.5. The top part of my HTML looks like this:

  2. <!DOCTYPE html SYSTEM "about:legacy-compat">
  3. <html>
  4. <head>
  5. <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
  6.     <meta name="viewport" content="width=device-width, initial-scale=1">
  7. <title>My title</title>
  8. <link rel="stylesheet" href="js/jquery.mobile-1.4.5.min.css">
  9. <script src="js/jquery-1.11.3.min.js"></script>
  10. <script src="js/jquery.mobile-1.4.5.min.js"></script>
  11. </head>
  12. <body>
  13. <div data-content-theme="c" data-role="page" lang="en-us">
  14. <div data-role="header" data-position="fixed" data-theme="c">
  15. <h1 class="title topictitle1">My topic</h1>
  16. <div class="ui-btn-left">
  17. <a href="index.html" data-role="button" title="Home" data-icon="home" rel="external" data-iconpos="notext"></a>
  18. </div>
  19. </div>
  20. <main role="main">
  21. <article role="article">
  22. <div class="body conbody"> 
  23. <section class="section"> 
  24. <figure class="fig fignone">
  25. <img class="image" src="C047CCBF1E02-low.png">
  26.  <p class="p">Definition of the serial number.</p>
  27.                  ...
  28.  
Thanks,
Mark