Another year later...luckily we got some more feedback even though it doesn't make the decision for a suitable default theme not easier.
The new master plan (suggestion): - merge https://github.com/geany/geany-osx/pull/35 to use it in Windows builds - integrate the Prof-Gnome theme from the https://github.com/geany/geany-osx repository (#3129) - set the default theme to Prof-Gnome in Windows builds (#3129) - update the new FAQ entry to reflect the new default theme and mention more of the alternatives mentioned in this issue https://github.com/geany/www.geany.org/pull/40
I'm aware that some of the contributors of this issue are not happy with Prof-Gnome and might prefer Adwaita, Boomerang, Monokai or whatever else. The comments above made clear we cannot find a theme as default to make everyone happy. To me it seems, Prof-Gnome will be a significant improvement over the currently used theme and it would make the default appearance of Geany on Windows and macOS more similar. Additionally, we have easily reachable instructions for the users on how to change the default theme on the website.