Thank you all for the precious advice! I found KoGa3's mods 
	
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 and Universal Ren'Py Mod (URM) 
	
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. I tried them both and by far the easiest is URM.
I also tried 
UnRen, and I de-compiled all the .rpy files with it; then I added the script that 
Abhai suggested in the relevant "init python" subsection, but it did not change anything. I suppose I should now recompile the .rpy files for the changes to take effect, but I don't know how, and I cannot see that option in the UnRen menu.
Anyway, thank for your replies, since now I'm deep in the rabbit hole of these mods, because I didn't know they could do these amazing things plus other that I dreamed of.
Cheers