Theme preview issue

Hi,
I can’t figure out why .header-img body class keeps being added in the theme preview in the theme directory. This class comes with completely different header styles but without an actual header image the header in the preview looks weird with huge spacing above and below which shouldn’t be there.

Currently this class is added this way:
if ( get_header_image() !== false ) {
$classes[] = ‘header-img’;
}

The page I need help with: [log in to see the link]