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,
add_css_js. And the add_action needs to be like this:add_action('after_setup_theme', 'add_css_js', 100000);wp_enqueue_style(), which allows you to define your CSS file's dependencies.