Chrome extension tooltip over text(one word) - google-chrome-extension

Question

I'd like to implement a tooltip dictionary chrome extension. When the user hover the mouse over a chunk or a word in text, It should show the block selection or change its background color and show the tooltip under the word.
How can I implement it?
I just did similar thing with background like this:
core.js
// Unique ID for the className.
var MOUSE_VISITED_CLASSNAME = 'crx_mouse_visited';
// Previous dom, that we want to track, so we can remove the previous styling.
var prevDOM = null;
// Mouse listener for any move event on the current document.
document.addEventListener('mousemove', function (e) {
var srcElement = e.srcElement;
// Lets check if our underlying element is a DIV.
if (srcElement.nodeName == 'DIV' || srcElement.nodeName == 'SPAN') {
// For NPE checking, we check safely. We need to remove the class name
// Since we will be styling the new one after.
if (prevDOM != null) {
prevDOM.classList.remove(MOUSE_VISITED_CLASSNAME);
}
// Add a visited class name to the element. So we can style it.
srcElement.classList.add(MOUSE_VISITED_CLASSNAME);
// The current element is now the previous. So we can remove the class
// during the next iteration.
prevDOM = srcElement;
}
}, false);
core.css
.crx_mouse_visited {
background-color: #bcd5eb !important;
outline: 1px solid #5166bb !important;
}