I have a collapsible set that I create on my website which looks similar to this:
You need to upgrade theme-classic.css
using jQuery Mobile ThemeRoller.
Place new theme and other dependencies in the following order
<head>
<link rel="stylesheet" href="css/themes/my-custom-theme.css" />
<link rel="stylesheet" href="css/themes/jquery.mobile.icons.min.css" />
<link rel="stylesheet" href="http://code.jquery.com/mobile/1.4.5/jquery.mobile.structure-1.4.5.min.css" />
<script src="http://code.jquery.com/jquery-1.11.1.min.js"></script>
<script src="http://code.jquery.com/mobile/1.4.5/jquery.mobile-1.4.5.min.js"></script>
</head>
Demo
Applying a theme changes the look and feel of HTML elements as here
div.setAttribute("data-theme", "c");
data-theme might be having a CSS class of its own which is effecting your webpage design
Are you using any CSS framework like BootStrap? Try adding this theme element to the exact div where you wish a design change