selenium: how do I Increase post width in wordpress blog

I've been trying to increase the width of the post container in my theme. The thing is that, I can't seem to get it working. I've tried editing my style.css, but it just doesn't work.

I have added a RIGHT sidebar via "STYLE" plugin.Everything seems fine. I have tried increasing the content widht.. didn't work. I've got a child theme, so, it's all good.

Here are the screen shots to my page :-