Currently Dojo provides no way of customizing the tooltip dialogs individually. One can modify the look and feel of all tooltip dialogs on the page, but not individually.
In this case, I have set class="myTheme" in the TooltipDialog, but this information is lost soon after the user clicks on the button to open the tooltip dialog. The next two pictures show the DOM as captured by firebug: one soon after the page load and second after the user once opens and closes the tooltip. The class name "myTheme" is present in the first screenshot (see <div id="f1" class="dijitTooltipDialog myTheme"), but is lost in the other one. Loosing the class name "myTheme" is the reason that the tooltip dialogs are not individually customizable.
