I mean this website: https://www.geany.org/download/themes/

Apologies for this, I think my pull request #57 is the problem. It wasn't obvious what to expect from "make index" according to the readme. Yesterday and today I spent time to investigate and fix it. I updated also the ADDING-A-THEME.md file, where I described the problem, so others don't come across it.

In short - the json index generation code is Python 2, while on some systems, mine included just "python" runs Python 3 and the code behaves very differently. I thought with minor tweaks I could make it run on Python 3, but modules behave different, so as a quick fix I downloaded the official Python 2.7 docker container and ran it there. Everything as I said is described in the MD file I updated and a new json index file was generated and added to the new pull request #59.

At some point the scripts must be migrated to Python 3, as Python 2 is officially "sunseted" as of Jan 1 2020 and is basically near dead.

Also it is good if all Python code sources are consistent, like for example to be formatted with the "black" tool (some scripts use 2-spaces tabulation, instead of 4 etc).