@vincent-legoll same thing but (for historical reasons) the default colour scheme lives in "filetypes.common" in the filedefs directory.
Change the line @shiftee said, but as you have inverted colours you make it it darker to make it lighter :)
Many colour schemes are [here](https://github.com/geany/geany-themes)
@shiftee changing "alt.conf" only works if you are using that colour scheme.