Need Help Centering Image in Header with Adelle Theme

Site link. I don’t know html very well and am having trouble centering an image in the header. It is showing as left justified. Below is what the code looks like:

/* Overall */
  .container {margin: 10px auto; width: 90%; overflow: hidden;}
  .header {position: relative; width: 90%; margin: 0 auto; padding: 40px 0 0 0;}
  .section {float: left; width: 60%; margin: 20px 0;}
  .aside {float: right; width: 35%; margin: 30px 0; background: #f8f8f8;}
  .footer {clear: both; overflow: hidden; margin: 0 auto; width: 100%; color: #777;}

  /* .header form */
  .header-form {clear: both; overflow: hidden; position: absolute; top: 4px; right: 20px;}
  .header-text {width: 80px; border: 1px solid #bbb;}

I would greatly appreciate if someone could help me out! Thank you!

-Brett