background color disappeared

You want the entire thing black? You will then need to change all your text to be white… not sure if that’s what you want … but

.site {
background-color: #000000;
}

should make everything black. You will probably have to modify other elements that may also have white backgrounds, or type that will be black that you’ll need to change.

Set up a child theme or use custom.css and do not make changes directly to your twenty-fourteen theme files.

Changing the .site tag as suggested by janet4now will work but leaves the centre block white near the photograph and text. Again, assuming you want the whole thing black you should also change the following to complete the look.

.site-content .entry-content, .site-content .entry-summary, .page-content {
background-color: #000000;
}

Thank you that worked!