Hi,

1/ You should paste this code into the function.php of the child theme or via the CodeSnippet plugin here: https://wordpress.org/plugins/code-snippets/

add_filter( 'body_class', function( $classes ) {

    if ( is_single() ) {

        $post_id = get_the_ID();

        // List the taxonomies you want to include
        $taxonomies = [ 'category', 'post_tag' ]; 
        // Add more if needed, e.g. 'penci_review_cat', 'topic', etc.

        foreach ( $taxonomies as $tax ) {
            $terms = get_the_terms( $post_id, $tax );

            if ( ! empty( $terms ) && ! is_wp_error( $terms ) ) {
                foreach ( $terms as $term ) {
                    // Add class in the format: taxonomy-term-slug
                    $classes[] = sanitize_html_class( "{$tax}-{$term->slug}" );
                }
            }
        }
    }

    return $classes;
});

2/ Now you can add CSS to filter the code for each category. For example:

.single-post.category-news .entry-content p:first-of-type::first-letter {
color: #000;
}
.single-post.category-solutions .entry-content p:first-of-type::first-letter {
color: #111;
}
.single-post.category-food .entry-content p:first-of-type::first-letter {
color: #222;
}

Regards,
PenciDesign.