This is indeed a matter for admins! I went to https://meta.discourse.org/t/code-block-line-numbers/330130, clicked the blue “Install this theme component” button, entered the name of my forum, and then confirmed that action from my forum interface. So far so good.
This feature was then listed as a Component (I guess that is technically a Theme Component) and reports version 0.0.2. But the dialog suggests that this artifact is currently considered an “Unused component”. And simple trials using the three ``` back‑ticks syntax did not render line numbers.
Is there something else I need to do? TIA.