Load CSS file in webview macOS

I am trying to load the local css file in WebView but after loading changes are not getting affected this is css file that i have downloaded and added to my project

 NSURL *scriptPathCSS = [[NSBundle mainBundle] URLForResource:@"dark" withExtension:@"css"];
 NSString *scriptCSS = [NSString stringWithContentsOfURL:scriptPathCSS encoding:NSUTF8StringEncoding error:nil];
  [_webView stringByEvaluatingJavaScriptFromString:scriptCSS];

the above block of code which i am trying.

Any help most appreciated.