Hi,
We don't add distro-specific information on the main README, mostly because that'd be a *lot* of information which might get outdated pretty fast without us knowing, and because such information are generally better stored in the dirstro's docs itself. However, we are happy to have distro-specific information or tips [on the wiki](https://wiki.geany.org/) -- feel free to add any useful info for Kali Linux.
This said, I'm wondering what the script you linked is really doing? I see that it *uninstalls* Geany from the distro's repositories, removes */root/.config/geany*, and then installs the Geany and Geany-Plugins packages from the same repositories. After that, it follows the geany-themes instructions for installing it. It eludes me why one has to uninstall Geany and then reinstall it from the same source? Also, removing */root/.config/geany* is questionable IMO: this directory most likely shouldn't exist, because we recommend against running Geany as root, but if it does exist, I wouldn't expect all users to want it gone. Finally, if Kali is not packaging geany-themes and you'd like it to, maybe you could ask them for it :)