[Velocd]

If you're not very familar with HTML, I recommend you learn the basics:
http://www.w3schools.com/xhtml/

The buttons are images, and if they hold the same filename as the original vB buttons, you just have to upload the images to the server (overwriting the previous buttons).

If they have different names, you'll need to edit the templates in the Admin CP finding the <img /> tag of the original buttons, and changing the path.

You cannot "highlight" buttons that are images upon hover (well, maybe with some very fancy CSS trick..). If the buttons are non-images, i.e. <input type="button" />, you can use Javascript to change button properties.

e.g.

<input type="submit" value="Click Me" onmouseover="this.style.background='#452456'" />

(I think that would do something, I don't often mess with mouseover's on buttons)