BG color to sidebar

I get it!
I put this in my css file:
.widget-area {
margin: 24px 0 0;
margin: 1.714285714rem 0 0;
padding: 12px;
background-color: #e6e6e6;

Which CSS file?

style.css

Do not edit the Twenty Twelve theme itself. First create a child theme for your changes. Or install a custom CSS plugin.