that would work only for the current user.
That's how GTK themes work on Linux, yes, but are multi-user Windows installations really that common on desktop PCs?
Even if, I won't modify the installer to modify the user's personal files. This is a no-go and should never be done. Never.
@techee's method sounds good and is much nicer.
How do we want to proceed here? I think there are two options: 1. leave the current Windows theming as is but document how to change the default theme (the howto is already available in https://github.com/geany/www.geany.org/pull/40) 2. include another theme in the installer and enable it by default
Option 1 would be the simplest and might suffice.
Option 2 would be better but we need to agree on which theme to include. If we do this, I would vote for @techee's variant of Prof-Gnome because it looks quite good (just my personal opinion of course) and more important we would have the same look and feel by default on macOS and Windows.