Remember to Backup before making any changes.

Just remove the blue lines and add the red lines.

I just notice the extra space. Know that look about right

I promise myself I will get some sleep.

Go to (includes/templates/Your_Template/css/stylesheet.css)

Code:
#navMainSearch {
         float: left;
         margin: 2em;
         background-image: url(../images/your_background.gif);
         padding: 0.2em;
         }

Change to:

Code:
#navMainSearch {
         background-image: url(../images/your_background.gif);
         width: 100%;
         font-size: 1.2em;       
         }
Used your background color. Example: (background-color: #9a9a9a;)

Or

Your background image. Example: (url(../images/your-background.gif)

If your width is 750px or any other number you can used that instead of 100%.

Go to http://www.browsershots.org to see how your website look on other browsers.

Validate your store: http://validator.w3.org/