How to add customized CSS variables for a theme plugin?

I want to define my CSS variables (like vendor/elgg/elgg/engine/theme.php) that I can use on other plugins (Elgg 3.2). How can I achieve that in my theme plugin? 

I saw the documentation about plugin hooks (vars:compiler, css) but cannot figure out how to trigger this hook. 

Thanks in advance!

Beginning Developers

Beginning Developers

This space is for newcomers, who wish to build a new plugin or to customize an existing one to their liking