The 'cls' property value allows you to set your custom class to the textbox component. The code below will change the colour of the inputing text to red.
<style type="text/css">
.myclass .textbox-text{
color: red;
}
</style>
<script type="text/javascript">
$(function(){
$('#tt').textbox({
cls: 'myclass'
})
})
</script>