refactor: Adapt to nbxmpp changes

Import Userrequested to merge
imprv-0392 into master

Namely, https://dev.gajim.org/gajim/python-nbxmpp/-/merge_requests/66

The change from background adaptation to just different lightness values is very approximate, and can be done to more precisely mimic the previous 80:20 mixing with white/black, if desired, but in general this is The Right Way™ to do it, methinks.

Edited by Import User

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