Remove Grey lines and large space above footer

Do not edit the Twenty Twelve theme. It is the default WordPress theme and having access to an unedited version of the theme is vital when dealing with a range of site issues. First create a child theme for your changes. Or use a custom CSS plugin.

Hi
I’ve got a child theme set-up. Which CSS style do I edit?
Thanks, Greg

The one in your child theme. I’d also recommend using Firefox with the Firebug add-on for this kind of CSS troubleshooting. Or use whatever developer tool is available in your web browser.

Hi
Is there any specific css class as I’ve changed some but it doesn’t make any difference?

please use one of the suggested browser inspection tools to find out where the formatting comes from, or possibly ask at a css forum like https://csscreator.com/forum