Print admin toolbar icon in custom module
If you want to add svg icon for your menu link item in admin toolbar, you have to alter toolbart.

In your mymodule.libraries.yml file

toolbar:
  version: VERSION
  css:
   theme:
    css/styles.css: {}

And you need to implement hook_toolbar_alter() in your mymodul.module

function mymodule_toolbar_alter(&amp;$items) {
  $items['administration']['#attached']['library'][] = 'mymodule/toolbar';
}