Thank you very much @dpoulton! It's good to hear that things still work because there's this official warning somewhere else in the forum that every CSS customisation can potentially become unusable after an update…
Strange thing is that contrast in the markdown editor seems to be better on smaller and sharper screens such as a 12" Surface. Large fonts on a 17" laptop tend to look rather grey and blurry, but maybe I should check first what colours are effectively encoded (after giving up my reluctance to CSS tweaks of course…).