UPDATED THE THEME SO IT WORKS WITH AUDACITY 3.x!
Same instructions apply as in OP, but you need to put the .png and .htm file in a subfolder of Theme with the name custom, like this:
ImageCache.png + ImageCache.htm to download:
https://mega.nz/file/tIYSjIaS#jgMqKjIDCqH7iLmtWgywyxJnegizpZkXTM5E82QtCJg
Example of the theme on Windows (but should work fine on macOS and Linux too):
