Changing css based on page?

I have a theme that I’m modifying by taking the sidebar off all pages except the front page. The content width is set to 500 pixels wide. When the sidebar is removed, the content moves to the left, but the css is still set to 500px wide. Is there a way I can create a variable to decide if the page is_front_page or not and to display a different div id so that I can make the width of the content change to 700px if the sidebar isn’t there? Thanks!