A few days ago, I contributed to creating a Colibre icon theme with some of its variants (SVG, light, dark) for Zotero . An enthusiastic person has done a PR (https://github.com/zotero/zotero-libreoffice-integration/pull/73) to add internal support some icon themes. Unfortunately this PR was rejected by upstream Zotero for a pretty good reason.
Departing from the discussion I recommend a feature for LibreOffice core to provide an API or convenience for extension developers to support icon themes automatically. That is, when the user changes the icon theme the extension automatically follows the icon theme that the user chooses. This may seem trivial, but it's great for increasing usability e.g. for visually impaired users can enjoy Sifr icon theme fully in addition to eye candy related stuff.
Step to reproduce:
1. Use Sifr icon theme
2. Install Zotero
3. Zotero still use "fresh" previous Colibre-like theme
As an extension developer myself, I feel this pain.
I'm setting this to NEW.