insert ad code above header

You mean above the top bar outside the container?

– Go to Frontier Options -> Widgets then check “Body”.
– On the Widgets page, add a Text Widget on the Body widget area.
– Add your Ad code on that Text Widget.

To center, add this on Frontier Options -> Custom CSS

.widget-body {
    background-color: transparent;
    border: none;
    box-shadow: none;
    display: block;
    margin: 0;
    text-align: center;
}

Note: If you edit any theme files without using a child-theme then your changes will be lost when you update the theme.