Help on CSS

I am trying to move my widgets to the middle of the page. I have no programming background but I still tried it by duplicating existing sidebars, placed them where I wanted and also edited some CSS code to match the format. Now, as the widgets are successfully showing off in the place I wanted, I want the white boxes containing ‘recent posts’ and ‘random posts’ should have ROUND CORNERS same as SIDEBAR and POSTS have.

How I do that?

Please Help