How to point multiple Grav sites to web-root CSS?

Hi! How can I point multiple Grav installations under same web root to fetch ONE 'custom.css' file? Right now in the twig partial, there's no way to get website root and put a custom CSS there only point to a custom file within the theme, or its there? Thnx!

just create a reference to the CSS with a hardcoded path. ie:

http://yoursite.com/css/custom.css

Thnx, I should have been clearer; we have to take in account http and https sites (dev and live) and also would like to know if there's a way of linking to root with 'assets.addCss' in partials?

Right now our dev site is http and the live site is https. This wouldn't be a problem taking care of with php, but I was under the impression there might be a twig way to get to site root?

{{ base_url_absolute }} should be the full base path with protocol.

Thanks for a fast reply! I'm getting 'website.com/blog/' with base_url_abs, but there are multiple Grav installations at 'website.com/', and ideally I'd like all those sites to share a 'custom.css' type stylesheet. This so I only have to compile one 'custom.css' file for all blogs. I guess I could move all Grav installations into one blog folder, but as of now the layout is 'website.com/blog01', 'website.com/blog02' etc. I thought there might be a way to address the very root of the site, not the separate Grav installation root. Maybe that's asking too much? The customer likes to have only one folder for CSS for the entire site, including all blogs. This to avoid confusion when updating. What would be the correct way ahead?