You can't. It's not a trusted event.
Events that are generated by the user agent, either as a result of user interaction, or as a direct result of changes to the DOM, are trusted by the user agent with privileges that are not afforded to events generated by script through the DocumentEvent.createEvent("Event") method, modified using the Event.initEvent() method, or dispatched via the EventTarget.dispatchEvent() method. The isTrusted attribute of trusted events has a value of true, while untrusted events have a isTrusted attribute value of false.
Most untrusted events should not trigger default actions, with the exception of click or DOMActivate events.
You have to add a class and add/remove that on the mouseover/mouseout events manually.