Can you get a posts category ID?

I want to make a different color which represents each category via CSS. To do this, I need to be able to get the category ID and add make that a class name.

I’ve highlighted in the post_html where I want the category ID to be.

<?php
if ( function_exists('wpp_get_mostpopular') ) {
    
    $args = array(
        'excerpt_length' => 200, 
        'title_length'=> 36,
        'post_type' => 'post',
        'limit' => 6,  
        'post_html' => '<div class="most-popular-box cat-[CATEGORY ID HERE]"><div class="wpp-excerpt"><div class="popular-box-thumbnail">{thumb}</div><p class="popular-box-title">{title}</p><a href={url}><p class="popular-box-summary">{summary}</p></a></div><footer>{category}</footer></div>',
        'range' => 'all',
        'stats_category' => 1,
	'thumbnail_width' => 300,
   	'thumbnail_height' => 60,
    'stats_category' => 1
    );
    wpp_get_mostpopular( $args );
}
?>

Is this possible to do?