Your text-container width could be a little wider, but your main problem is that your colors are terrible.

Day-mode background could be anything from off-white to financial-times color (ft.com). Text color should be a dark grey.

If you want a nightmode, fire up Visual Studio Code and take notes.

vitalik has a pretty decent day and nightmode. https://vitalik.eth.limo/general/2023/12/28/cypherpunk.html

sam altman demonstrates how nice grey text is. https://blog.samaltman.com/