/* @import url('http://example.com/example_style.css'); */

/***********************************************/
/* CSS @imports must be at the top of the file */
/* Add them above this section                 */
/***********************************************/

/*****************************************/
/* Start your style declarations here    */
/*****************************************/

<style>
  header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    width: 100%;
    z-index: 999;
  }
</style>