What you are asking for is no simple css change. The search widget in that layout of the Jewlerybox theme is absolutely positioned to the right of the header controller and all the widget icons a margin-ed and set to the left to make room for the search dijit to be there. Your desired change would involve trying to figure some js code to get the center of the app and then half the Search widgets width (275 px) and setting the search widgets left property to that value and also setting the padding/margins of the other header components to account for the search dijits position. You would have to update this in code each time the browser is re-sized. This has my head hurting even as experienced JS developer.
Thanks for the response. What you are saying makes sense. I was hoping to be able to grab a theme that was similar to what I needed and tweak it. It's now clear I'll have to develop my own theme with a header. I can then mess with resizing the search widget. Kind of like this example I just found:Interactive Map - County of Simcoe (GIS)