TwentyTwelve modify sidebar content size

Hello everyone,
I’m more or less new on wordpress so so I apologize if I ask stupid questions.
My problem is that I change size of the sidebar and thereby increase the size of the content on front page. To do that I use an css

margin: 0 -1% 0 0;
width: 100%;
}

#content {
margin: 0 34% 0 1%;
width: 120%;
}

#secondary {
float: right;
margin-right: 1%;
width: 18.8%;
}

But now my full-page template have too big content size, does anyone know how to change code that css would be in use just for front page.

Thanks for all answers and time