border around button

Hi!
Please check http://interiortrash.com/wp/interior-trash/
There’s a button (“About us”). Background should be white, text color black. So far so good. But.. I would like to add a black border (just like on the column above the button (with text “the idea is not…”).
Is there a way to create this? There will be a lot more of these buttons and they all need such a border.
Thanks for your help!
Martine