Adds a class name to the Event Espresso event post's post class so events that belong to a specific category can be uniquely styled with CSS. Requires Event Espresso 4.2.

ee_add_taxonomy_post_class.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

// Add a class name to the event post's post class that's based on the category it's assigned to

add_filter( 'post_class', 'ee_add_taxonomy_post_class', 10, 3 );

function ee_add_taxonomy_post_class( $classes, $class, $ID ) {
    $taxonomy = 'espresso_event_categories';
    $terms = get_the_terms( (int) $ID, $taxonomy );
    if( !empty( $terms ) ) {
        foreach( (array) $terms as $order => $term ) {
            if( !in_array( $term->slug, $classes ) ) {
                $classes[] = $term->slug;
            }
        }
    }
    return $classes;
}