When you drag and drop a UI component, the Properties panel will show you properties you can set. For instance, here is an example when list item is selected:

Sometimes not all properties might be exposed in the app builder, but it doesn’t mean you can’t set them. Using More Properties feature, you can set any properties supported by the component HTML or jQuery Mobile component.

For example, you want to change the list item icon to something list (no icon option is available but coming soon). With the list item selected, click More Options and add add the following property:

The icon will not update in design time, but when you run the app, the first icon will change to bullets:

To make this work, you need to know the property name and possible values. This is easy to find in jQuery Mobile docs or HTML docs.