when I design a page that has dark color background, I originally use white color text (hex FFFFFF). then I find some others great work use soft white on dark background (like hex: F5F5F5 etc). I find some answers about it says this can decreases eye strain and strengthens reading stamina.
but how to decide which soft white color to use of text on dark background, is there some general rules of picking text color or just decided by feeling?