Change coursepress icon to a cutom one


I want to change the coursepress icon to a custom one I created. Tried to do it with admin menu editor but no effect.

I also tried to do it by following this article:
but it doesn't take effect in admin menu bar.

I see it has these attributes,
CSS class: menu-top toplevel_page_courses
Icon URL: ...../wp-content/plugins/coursepress/images/coursepress-icon.png
Hook name: toplevel_page_courses

I prefer to do it with some css rather than changing the plugin files because a plugin update would change all those customizations.

Also please help me with my custom icon as I don't know the proper image dimension, quality, etc...
If adding a custom icon is too much work, I would settle with adding Wordpress Dashicons: