Looks like the default is
#header-widget-area {width:50%; right:0px}
If you need to find the CSS code (class or ID) the best way to do this is in your browser: right click the item in question (ie the search box), select “Inspect Element” and there you will see the CSS code for the element, you can even edit the CSS right there and then in the browser to see what effects the changes have, and see which one does what you need, then make the required modifications in your own CSS.