mnemmixip

You can change the position and more options, redeclaring in the styles.css of your theme

In your case, add in styles.css:

body.custom-background {
background-position: top center;
}

If you want responsive background, add

body.custom-background {
background-size: contain;
}

Easy…. just edit the style.css of your theme and add

body.custom-background {
background-size: contain;
}