An approach to Themes

I am wondering if this might be a good place to share these thoughts. Maybe, this is all clearly understood by others or laid out somewhere but it still escapes me.

I know Drupal is more like clay and can be shaped in such versatile array as one might imagine, but in my learning path I find myself trying to understand Drupal in simpler terms like a Mechano set.

When it comes to themes, starting first from a technical perspective, where might one start?

My understanding is the theme layer is like a skin over the functionality provided by core and module layers and the way they are configured. And of course Panels is somewhere in that zone where the functionality and skin meet.

The theme layer consists of essentially two layers, the base theme and the overrides.

Well, looking at the overrides we find they aren't just a fews tweaks, but a syntax for defining any possible override you might imagine, not just for nodes but also for views. And that overrides can occur by way of either .tpl.php files or css overrides. And these override files may reside in contrib module directories and be copied to custom theme directories for alteration. Is it any wonder it took a while to understand.

Okay, but just for the custom theme overrides, is there a way to place all the custom theme overrides in one place so they apply to all themes enabled for the site rather than needing copies of all the override files in each theme directory?

Yes, there might still be some theme specific override that might just have to reside in the theme folder but having one set of override files instead of multiple copies in different theme directories would remove redundancy. Can this already be done through some configuration?