Customize Buttons

Hey, I'm new to Elgg and I really have no knowledge of how to make themes or plugins for Elgg yet, but I was wondering how I would change the backgrounds of the buttons (I really don't like the glossy look on the buttons). Is there already a plugin to do this or if they are in a folder somewhere in the Elgg (I've already checked the '_graphics' folder and nothing..)

Please help!