There isn't much to it really. The controllers are "#bar", "#addButton", "#msgButton", "#tradeButton", "#ignoreButton" respectively. The #bar is the wrapper for the other buttons, it holds them and acts as a background etc for them. You can change the dimensions ( width property, height property ), overflow property, background property and the position property ( coupled with top property and left property ). Margins and paddings can also me altered. ( You can actually do anything you want to any property via C.S.S, its all according to what the end result you are trying to get. ) The same applied for the actual "Buttons". Please keep in mind that the selectors are case sensitive meaning that "#Addbutton", "#addButton" and "#addbutton" are not the same thing. =) To change the actual button icons simply specify a background property on the selector you want. As for the "on hover" the ":hover" class is a universal hover property that C.S.S uses to tell the browser to only initiate the code on hover of the section it is used on.
An example of the hover use would be
Code
selector
{ background: red;}
selector:hover
{ background: green;}