Get parent category class in post_class()

Perhaps something like:

function mytheme_get_post_class_parent_cats() {
    $parent_cat_classes = array();
    // Get parent categories
    $post_cats = wp_get_post_categories();
    // Loop through post categories
    foreach ( $post_cats as $post_cat ) {
        $cat_parents = get_category_parents( $post_cat , false, ',', true );
        // if there are any parent categories
        if ( $cat_parents ) {
            // create an array
            $cat_array = explode( $cat_parents, ',' );
            // First key of the array is the top-level category parent
            $parent_cat_id = $cat_array[0];
            // Get the name of the top-level parent category
            $parent_cat = get_cat_name( $parent_cat_id );
            // create the CSS class .category-parent-{categoryname}
            $parent_cat_classes[] = 'category-parent-' . $parent_cat;
        } else {
            // Otherwise, the category is a top-level cat, so use it
            $cat_name = get_cat_name( $post_cat );
            $parent_cat_classes[] = 'category-parent-' . $cat_name;
        }
    }
    // Return CSS class, or false
    return $parent_cat_classes;
}

Then, when you call post_class():

<?php post_class( mytheme_get_post_class_parent_cats() ); ?>

• Codex ref: wp_get_post_categories()
• Codex ref: get_category_parents()
• Codex ref: get_cat_name()