There are different ways to implement such a menu.
In tersus.web (a copy of which can be downloaded from http://www.tersus.com/#Id=139), it is implemented using the tersus.web/Main Page/Navigation List/Navigation List model which contains a <ul> tag containing <li> tags (1 for each menu item) which in turn contain an image and a <span> tag with an <On Click> event handler. This model combained with the CSS and images provided in the project's web folder result in the horizontal menu.
Bear in mind that this is a rather old model, and there might be other/better ways to reach the same result.
I'm not sure how all this relates to Callable Service, so please elaborate if you feel it necessary.
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