Do you mean, rounded corners?
Small rounded corners on the forum board
Then you have to chance the global.css of your theme. F.e. go to #panel and ad the next code to your Extra CSS Attributes:
border-bottom-left-radius: 10px;
border-bottom-right-radius: 10px;
border-top-left-radius: 10px;
border-top-right-radius: 10px;
border: 1px solid #e4e4e4;
margin: auto auto;
This is just a example, there are more files that you have to chance