it is quite a challenge to keep track of possible customization, same as for finding inspirations.
How about creating a continuously maintained gallery showing a best-of (in the Theme Creator)?
So far I have come across Looking for examples of some nice customizations (I want my community look pretty;)), what led me to nice examples like New Relic, FeverBee Experts and Revolut.
This topic isn’t complete as it misses Angus’ Sandbox what shows nice examples of the Topic List Previews and comes with kind of news blog. Another nice example of the Preview plugin can be found in Any live examples of category-specific CSS styling?.
In addition, a reference to Material Design Experiments is also missing.
An absolutely astonishing example is shown in Sharing our experience changing the Discourse UI but no details are given .
I made the experience that documentation on #themes isn’t centralized. It is done well for the Material Theme on Daemonite Material Discourse Theme. Moreover I found some simple overview on Introducing Discourse User Themes and some on Discourse Interface Tweaking and User Feedback.
External coding sources are also rare. So far I found only Customizing a Discourse Forum for Non-Coders and Designers - DZone Web Dev. but the documentation in Developer’s guide to Discourse Themes is extensive.