Why is my wp_enqueue_style() not working?

Looking at your debugging output it seems like you might be developing a child theme? If so then the function get_stylesheet_directory_uri() is needed instead of get_template_directory_uri() because the latter will always use the Parent theme, whereas the former will use the Child if its present.

So try this instead:

function load_stylesheets(){    
        wp_enqueue_style('bootstrap', get_stylesheet_directory_uri().'/assets/css/bootstrap.css', array(), false, 'all');
        wp_enqueue_style('style', get_stylesheet_directory_uri().'/style.css', array(), false, 'all');
    }
add_action('wp_enqueue_scripts', 'load_stylesheets');

The other errors in your debugging are unlikely to be a cause of the missing CSS but if the above doesn’t work, please paste in part of your functions.php as per my comment above.

Update. I setup a new install of WP and applied your function above, dropped in the Bootstrap 4 download files into the correct folder to simulate your setup and added the style. Nothing more than that. Everything worked fine and the background turned to Black as you’d expect.

So I think the WordPress side of things is fine, and its possible that even the CSS setup is correct.

I think there are two further things you could check:

  1. ensure you can see your theme style.css appearing after/below the Bootstrap css in the head. It must be after/below else it will be overridden (which looks fine in your screenshot tbh).
  2. use the Inspector to see what your CSS is looking like when used by the browser. This might differ from what you expect.

In Chrome – Right click and inspect page to open DevTools. Switch to the network tab and apply the CSS filter at the top. Reload the page, and you should see all CSS files in use listed there. Click on your Style.css and you can see a preview of it on the right. Is it populated with the correct code?

Beyond this, I’m not sure I/we can help much more as the files appear to be correctly enqueued in the head.

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