The load-style.php URL you provided above is redirecting to the login form and this is why the inspector is complaining about a HTML response.
I tested this on a fresh installation and WordPress returns the styles even if you are logged off so you much have some .htaccess or PHP code redirecting the load-style.php file to the login form.
You have also removed a line of the core which tells the browser the PHP script is a CSS file, though I suspect Chrome would still parse it so long as the response was actually CSS, you should never do this and as such I would also recommend re-installing WordPress via Dashboard => Updates to get the core back to its correct state.