Jekyll Syntax Highlighting Not Working - Classes Are Not Being Added

Syntax highlighting not working in MD files for Jekyll?

Your Markdown file syntax is fine, you don't need to change it (check my comment bellow)

To get the code highlighting working, you need to check 2 things:

the highlighter and the markdown processors are correctly configured in _config.yml
the generated html files have access to a CSS highlighting-syntax style rules

1. Highlighter and the markdown processors configuration

As of jekyll 3.0, Kramdown as the Markdown engine, and Rouge as the syntax highlighter. are the default jekyll setting, and the only setting supported by github pages.

So you can remove their related setting, or set it explicitly at _config.yml as follows:

# Conversion
markdown:    kramdown
highlighter: rouge

# Markdown Processors
kramdown:
  input: GFM
  auto_ids: true
  syntax_highlighter: rouge

2. Code highlighting style:

the generated html file should have access to a CSS code-highlighting rules, that depends on the theme that you're working with.

One way to do so, is to have a code-highlighting style rules defined in the main css file, then include that file in the html head of the default layout.

define the CSS code-highliting rules

Make sure that the main CSS file, (located at /assets/css, and usually named main.scss or style.scss) has some code highlight CSS rules defined, either explicitly defined there, or by importing a file (scss, sass, or less) that contains the CSS rules.

for a quick check, I've putted some scss code-highlighting themes in this repo

clone sass-code-highlight repo
put sass-code-highlight folder inside the sass directory (by default: _sass)
inport the code-highlight to the main css file

in assets/css/main.scss add the following:

    @import "sass-code-highlight/monokai"; // 'monokai' as example

include the Main CSS in the HTML HEAD

you need to have the sinppet bellow in the default layout (_layouts/default.html)

<head>
  <!-- head stuff-->

  <!-- CSS -->
  <link rel="stylesheet" type="text/css" href="{{site.baseurl}}/assets/css/main.css"> <!-- IMPORTANT -->
</head>

either directly, or by including a head.htmlfile - located at _includes directory - into it, as follows:

<!DOCTYPE html>
<html lang="en">
  {% include head.html %} <!--  <- include the head -->
  <body>
  {{ content }}
  </body>
</html>

Note: make sure that the css path is valid.

Jekyll, Github Pages: syntax highlighting not working

The link address in _layouts/default.html was incomplete: it didn't include {{ site.baseurl }}

The full line should be:

<link href="{{ site.baseurl }}/css/syntax.css" rel="stylesheet">

I can't change jekyll syntax highlighter at my github blog

You're mixing up the command line instructions with the jekyll instructions. Also it seems that your command rougify style monokai.sublime does not work properly as if I look at the CSS file in your blog it says unknown theme: monokaisubline (https://azzyjk.github.io//assets/css/syntax.css).

So easiest solution just c&p one of these CSS stylesheets into your style.css files https://github.com/jwarby/jekyll-pygments-themes.

Here are the Jekyll docs https://jekyllrb.com/docs/liquid/tags/#code-snippet-highlighting

Edit:

As CSS is now correct but the correct CSS classes are not applied to your code snippets, can you try to set your syntax highlighter in your jekyll config.

kramdown:
  syntax_highlighter: rouge
  input: GFM