The jQuery UI theme css file includes an edit link in its comments. This link opens Themeroller with the settings of your custom theme.
/*
* jQuery UI CSS Framework
* Copyright (c) 2009 AUTHORS.txt (http://jqueryui.com/about)
* Dual licensed under the MIT (MIT-LICENSE.txt) and GPL (GPL-LICENSE.txt) licenses.
* To view and modify this theme, visit http://jqueryui.com/themeroller/?ffDefault=Segoe%20UI,%20Arial,%[...]*/