I want to create a custom theme with custom cursors on Ubuntu 20.10, but all tutorials/instructions I've found are at least 10 years old and don't work. Any tips?
It can't answer your question in detail, however, the following approach might be useful to you.
If you take a look at gnome-look.org you'll find the "cursor" theme section on the second position to the left side. In the provided link the author of that particular cursor-theme (submitted on 2020-11-15) and therefore fairly recent mentioned in his description below the preview:
- clickgen is (an) API for building X11 and Windows Cursors from .png files. clickgen is using anicursorgen and xcursorgen under the hood.
- Puppeteer is a Node library which provides a high-level API to control Chrome or Chromium over the DevTools Protocol. Puppeteer runs headless by default, but can be configured to run full (non-headless) Chrome or Chromium
So you might look into those two applications and/or directly get in contact with that community for detailed information - who better is there.