I failed to find a .gtkrc-2.0 file after doing a global search, and after looking in the location suggested by the [Geany manual](https://www.geany.org/manual/current/) - namely the "etc" subdirectory of the Geany install folder (or any other).
I also tried creating a file containing the line noted by @eht16 and saving as ".gtkrc-2.0" to the "etc" subdirectory, which also had no effect. Admittedly, I'm unfamiliar with these files and their structure, so the probability of a PEBKAC error is high.