Change right margin in right content, left sidebar layout

In the Twenty Eleven theme, using a child theme, I have tried every suggestion for changing the right margin/padding in the left-sidebar, right-content layout. I piddled mostly with the below:
#primary {
float: left;
margin: 0 -26.4% 0 0;
width: 100%;
}
#content {
margin: 0 34% 0 7.6%;
width: 58.4%;
}
#secondary {
float: right;
margin-right: 7.6%;
width: 18.8%;
}

It didn’t work. And yes, I tried several adjustments that resulted in the total equaling 100%. It still didn’t work.

The website is http://www.lesliemweller.com. I’m trying to get the “fern” image to shift to the right margin next to the outer gray border. If this also causes text to butt against the border, that will also need to be fixed; so that would be another question.

I spent about 12 hours searching for an answer to this and tried endless suggestions that apparently worked for some users, but did not work for me, so please don’t ask me to first search. This is my last resort before using another program – or handcoding HTML. Thanks.