Trying to get my CSS files to load last using add_action(after_setup_theme, add_css_js, 100000)

Sorry I am unable to comment so I have to post here.

I will give this answer as some examples and then hopefully you can use in your situation or provide further information about your exact setup.

Make sure you are using a child theme (If you are unsure about what this is please read this

In my sites I find I am using storefront a lot so an easy way to start off with child themes would be to give this one a try

Ok so now that we are on the same page in your Child theme functions.php add or un comment the following:

/**
 * Dequeue the Storefront Parent theme core CSS
 */
function sf_child_theme_dequeue_style() {
    wp_dequeue_style( 'storefront-style' );
    wp_dequeue_style( 'storefront-woocommerce-style' );
}

Once you have that done you can add any css to your stylesheet (style.css) or even add css from within your function pages.

I really hope this helps!

If not please provide details of the theme and the function you are trying to use / call.

Cheers,