You don't need to add additional markup to source code and neither to specify the language of source code snippets since prettify will guess. Highlight.js extension for Joomla is compatible with WYSIWYG editors and includes several themes to customize the colors and styles of source code snippets.
Out of the box, CedHighlightjs for Joomla comes configured to automatically perform syntax highlighting of source code snippets in <pre> or <code> tags on your Joomla pages. Automatic syntax highlighting mode is pretty simple, but powerful at the same time. Several settings can be configured at: Administration - Plugin - CedHighlightjs
CedHighlightjs includes several CSS styles publicly available which are used as themes for code Highlight.js