Dear custom theme creators and maintainers,
I recently wrote some new instructions on how to create custom themes: Creating custom themes · audacity/audacity Wiki · GitHub
You’ll note that the recommended method no longer involves editing the imagecache.png directly, but rather generating it from a bunch of individual files (PNGs for the icons, with a TXT file for the colors).
I highly recommend you convert existing themes to use this method. In the medium term, the entire idea of an imagecache is going to be scrapped in favor of doing it the way it’s done in the source now. So at that point you’d just put the individual PNGs and the one TXT into the custom theme folder and load that directly.
Update, July 29: The new approach may completely replace the old one by the end of 2023.