QUESTION Why are components under 'Theme' tab

[vanfruniken]

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For site wide injection of these things, we use plugins or dynpages plugin to load components.
For advanced asset or code injection WITHOUT writing a plugin, use my "hook components" plugin.

Which basically lets you map components to plugin hooks

Thanks, shawn_a for pointing out the capabilities of your hook components plugin. It rocks! (in particular the hook_theme-header, which, contrary to what its name might suggest, is NOT theme-specific).
In conjunction with dynpages, I am now able to include site specific business logic into my site, which is robust w.r.t. theme switching.