Only the project theme is included during rendering, no matter what components you use from a package. Any package theme is not used, only when viewing the package project. You will have to import (copy-paste from the package) the variables you need into your own theme. This is probably something we want to automate as you install packages, but it is currently not a feature.
Keeping everything in a single static theme makes a lot of things more simple than dynamically merging different themes. If you have some good idea of how you would like to see package-themes work different, I think we are open to suggestions as it is the last part of theming that is not complete.
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