@Ken Maltby
Thanks for your informations. On my version of koreader, this file is named "defaults.lua". Following your advice, I modified the settings (gamma to 35 for default...). It did improve somewhat the sharpness of the font and - for me - the quality of reading.
It's still not on a par with the kind of emboldening we can get
this way. But it looks like it's a trade secret.