The Vim plugin seems to display the background as the specified colour. Normal styling in Scintilla allows for only 255 styles, so thats not enough so AFAICT that may not be possible without Scintilla additions since it can only specify foreground (ie the text) colours with RGB for indicators, see [here](https://www.scintilla.org/ScintillaDoc.html#SCI_INDICSETFLAGS). Then a suitable Geany plugin could do it.
Scintilla is the editing widget, it is a separate project and any changes would have to be made there first.