You could easily override the styles for every select menus in CSS:
div.ui-select span.ui-btn-inner {
border: 1px solid red;
color: red;
text-align: right;
}
.. or if you want to stick to jQuery:
$('.ui-select span.ui-btn-inner').css('background-color', 'blue');
.. to customize a specific select menu, give your select element an id and then to select it with jQuery you'll need to append -button to the id. That's due to the fact that jQuery Mobile creates multiple elements that you might not know of. So, if you'll give your select menu an id custom-id and you want to change its background color, your jQuery should look like this:
$('.ui-select #custom-id-button span.ui-btn-inner').css('background-color', 'green');
.. and same in CSS:
div.ui-select #custom-id-button span.ui-btn-inner {
text-align: left;
}
Updated JSFiddle
http://jsfiddle.net/5Bkx9/6/