I got to probable candidate - STYLE_LINENUMBER, while searching /src/ folder for "line.?number"
and file "highlighting.c:798":
set_sci_style(sci, STYLE_LINENUMBER, GEANY_FILETYPES_NONE, GCS_MARGIN_LINENUMBER);
Any comment should I look elsewhere or so?
I don't know C/C++ to find my way
As I build Geany and plugins from source, if it's not much trouble and some dev can give me a tip how to change Geany source to make linenumber one point smaller then rest of editor font, I'd appreciate it much.