The default unfocused selection style is part of [Scintilla](https://www.scintilla.org/), that is another project, its not Geany code. Geany uploads Scintilla code unmodified, we do not have the resources to maintain patches for it.
You may try to have Scintilla change that default, but since the current Scintilla version has an API for setting it, I doubt it will be accepted.