I discovered that it's possible to switch color theme on Geany, since the version 1.22 (we are currently using v 1.38). Generally speaking, a dark theme is more comfortable for the eyes, it is somehow less tiring to be working for several hours in front of a screen when using a dark theme. Furthermore, for people using an OLED screen there are noticeable advantages regarding power consumption and pixels life span.
To change the Geany color theme, do the following:
1) go to here
2) click on the Download link which corresponds to the theme you would like to use
3) select and copy in memory all the text which the file is composed of
4) with a file manager go to the folder /root/.config/geany/colorschemes
5) here create a new empty text file, name it as you wish and give it .conf extension
6) open the file you just created with Geany and paste herein all the text you copied in RAM in step 3. Save the file
7) Close and re-open Geany
8) From the menu bar, choose “View” → “Change Color Scheme…“, and select the theme you just added
9) That's it. For installing a dark theme, just choose one of the many dark themes from the Geany themes web page