如何在AJAX调用中引用.CSS？

My goal is to add dark theme in slack windows application. I found so many threads which recommend some solution. Here, they refer to some CSS and that works fine.

Code that works:

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'https://cdn.rawgit.com/laCour/slack-night-mode/master/css/raw/black.css',
   success: function(css) {
     $("<style></style>").appendTo('head').html(css);
   }
 });
});

I do not want to access these links due to security reasons so copied that CSS file on my machine and want to access that but I guess I am doing something wrong.

Following are my attempts:

Attempt-1

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   href: 'C://LocalPath/slack/app-3.3.1/resources/app.asar.unpacked/src/static/slack_dark_theme.css',
   media: media || 'screen',
   type: 'text/css',
   rel: 'stylesheet'
   success: function(css) {
     $("<style></style>").appendTo('head').html(css);
   }
 });
});

Attempt-2

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'C://LocalPath/slack/app-3.3.1/resources/app.asar.unpacked/src/static/slack_dark_theme.css',
   dataType:"script",
   success: function(css) {
     $("<style></style>").appendTo('head').html(css);
   }
 });
});

Attempt-3

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'C://LocalPath/slack/app-3.3.1/resources/app.asar.unpacked/src/static/slack_dark_theme.css',
   dataType:"script",
   success: function(css) {
     $("head").append("<style>" + css + "</style>");
   }
 });
});

Attempt-4

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'C://LocalPath/slack/app-3.3.1/resources/app.asar.unpacked/src/static/slack_dark_theme.css',
   dataType:"script",
   success: function(css) {
     $("<style type="text/css"></style>").appendTo('head').html(css);
   }
 });
});

Attempt-5

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'file:///C://LocalPath/slack/app-3.3.1/resources/app.asar.unpacked/src/static/slack_dark_theme.css',
   success: function(css) {
     $("<style></style>").appendTo('head').html(css);
   }
 });
});