you can also use :host, which target the custom element specifically (and using :host on custom-element children does not, from what I recall off hand, get broken when nesting) and then target its children.

Also, css variables can be global and still enter into the shadow-dom unobtrusively (This is without using the polyfills, though, I believe this is a limitation in the polyfills currently, which might be why that part of it has been confusing in the past.).