[Theme: Pinboard] Change footer widget width with CSS

Hello fellow forum users,
I’m trying to customize my blog using a child theme based on Pinboard theme as a parent. I managed to create the child theme sucessfully and now I need to do some tweeks.
I used the devtool (ctrl+shift+i) on google chrome and changed my footer sidebar to look like this: http://worlduponmyshoulders.com/wp-content/uploads/2014/10/footer.jpg and here are the changes I’ve made: http://worlduponmyshoulders.com/wp-content/uploads/2014/10/edit.jpg
I know that it’s possible to edit the child theme just by adding some lines on the style.css file through the appearance>editor menu or by editing a .php file and copying it to the child theme folder, but I have no idea where and how to do it.
My page/blog is http://worlduponmyshoulders.com
Can anyone help me?