how to add border to sidebar images?

Hi. I'm trying to figure out how to add a border to any buttons I add in the sidebar (just one button so far, but I'll be adding more eventually). I'm sure it's just a simple bit of CSS to add, but I can't figure out what exactly. I'm trying to add a solid 1px #000000 border, so any button I add to the right sidebar will automatically have it. Help?

Thanks!

Riin