Designer 2 cents here: Contrast is needed to reduce eyestrain, actually the problem with light mode is colour in code editors, colour is better perceived on a dark background. But if you a reading long form text with no colour in general you will have better contrast on a white background. If you are using light colour theme in your editor, try to tame the whiteness of your background, 80 to 70% grayscale (you can warm it up with a little yellow tint). The next important thing is your font selection and linespacing.