That is something I would like to try out for quite some time now and the current theme encourages this so I gave it a couple of minutes and 15 minutes of CSS later I came up with something readable. I activated it for my English readers to visually differentiate the language as part of my reflexions to tackle that issue on the same website. Obviously it is only a part of the answer to address it.
Let me know if it is a bad idea because it lacks some context or the contrast is kind of hard to read for you or whatever before I’m going too far down that path. Another option would be to more clearly separate English and French contents to avoid going back and forth on dark/light themes if you read both.
Who knows, maybe one day I will switch the entire site to a dark theme :-).