1- First of all give an id to your theme style sheet link.
<link id="theme_style" rel="stylesheet" type="text/css" href="http://www.jeasyui.com/easyui/themes/default/easyui.css">
2- Add following javascript code.
<script>
/*declare variable to store theme style sheet links*/
var theme_styles = {
'default': 'http://www.jeasyui.com/easyui/themes/default/easyui.css',
'black': 'http://www.jeasyui.com/easyui/themes/black/easyui.css',
'gray': 'http://www.jeasyui.com/easyui/themes/gray/easyui.css'
};
/*function to change themes*/
function changetheme(themname){
$('#theme_style').attr('href', theme_styles[themname]);
}
</script>
3- Calll the changetheme function by passing your theme name as parameter .
Please see attached example file.