How to update Themes?

[zook2]

I asked Ranran about missing icons in the Line Manager:
https://forum.simutrans.com/index.php/topic,21455.0.html

Today a brief survey of all players logged on to the Transshipment server turned out that 100% of those surveyed (2) had never seen these icons.

The files in my themes folder are almost all from August '21. I can't find the files on Github and I only use the old manual download link to the Win64 zip (in the "How to download Simutrans" thread). How do I update themes?