How to change Expand and Collapse images of ZK Grid hierarchy Component

Today i got a new POC(Proof of concept) task how can we change the default expand and collapse images of ZK Grid hierarchy Component as i am not much knowledge of CSS i am not sure how we can do this. Its clear to me we have to make change in CSS to achieve this but in which css class i have to made the changes i am not sure then i have to ask this question Ben bai a zk developer ,very well and deep knowledge about ZK,CSS,Jquery etc. He told me to write this below CSS to achieve this



Here is the ZUL file for this ..


Note:- As i am not sure how we can get which CSS apply in which component @Ben suggest me a better way to do this in Google Chrome Browser ,What we can do is right-click on the expand/collapse icon in Chrome then click 'Inspect element' to view the dom elements and its css style in Chrome console