Changing Menu and Main Background Colors

In your Child Theme or CSS Editor Plugin try this code:

Navigation background

.main-nav, .main-nav li {
    background: #009900;
}

Just change the green color I used.

For the background:

Appearance > Customize > Colors > Background color

If you mean the content background color:

.mh-wrapper {
    padding: 20px;
    background: #fff;
}

*Note
If your theme has a custom CSS Editor in it, you can also paste the code in there.

Never edit core CSS or PHP files under Appearance > Editor. Your edits will be deleted when the theme is updated.