add image size still doesn’t work even after regenerating thumbnails

There are a couple of things to check here.

First, make sure that add_theme_support( 'post-thumbnails' ) is loaded before add_image_size( 'small-thumb', 60, 60, true )

You can always hook everything through a function to the after_setup_theme hook. I always add these in my theme setup function

function wpse_setup_theme() {
   add_theme_support( 'post-thumbnails' );
   add_image_size( 'small-thumb', 60, 60, true );
}

add_action( 'after_setup_theme', 'wpse_setup_theme' );

Apart from that, everything should work if you call your post thumbnail correctly in the loop.

On your question

Does CSS styling like max-width/ max-height/ width / height or
anything effect WordPress’ thumbnail functions?

No, it doesn’t. CSS only manipulate how a thumbnail is displayed on the front end