The online sandbox is only for demo purposes and you can not access its App_Data folder in order to create a Theme that will allow you to add css files which will modify the navigation. You can use the built in skins
in the navigation control to style the menu. To apply them you go to edit you navigation widget expand the appearance and styling and enter the name of the skin in the Css Wrapper class box.
To achieve the changing of each menu item you can download latest sitefinity release and create a project on your local computer.
Then referring to the Theme creation documentation
you create a theme and register it.
Then you apply the theme to a template and use that template to create a page.
Note you put your css styles that will style the navigation widget based on its already created classes. Navigation widget uses RadControls to display the different types of menus. For example you review the navigation widget with firebug and observe the css that it adds. You can then add or overwrite the styles using !important.
Other way of doing this is to drop a CSS widget on the page you want to style the menu or at a template and reference css files or add inline css styles that style the menu.
I hope this is helpful to you. If you have more questions please write back.
the Telerik team
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