How do I add a custom CSS file for default theme? For example - I have enabled Zircon as my default theme. I want the user menu account to appear on left. 
I want to add a custom CSS file where I can write a CSS elements and add to a default theme on site.
In Drupal 8, stylesheets (CSS) and JavaScript (JS) are loaded through the same system for modules (code) and themes, for everything: asset libraries. Asset libraries can contain one or more CSS assets, one or more JS assets and one or more JS settings.


More details on 
https://www.drupal.org/developing/api/8/assets