You can either select a light background (my preference) or, as Wayne mentioned, use the mouse to highlight the text that's hard to read.
Not everyone (hopefully fewer and fewer) uses the default dark background.
I'm 58, but my eyes are only two.
I've never changed from the default "Dark Grey" background with "White Font". Works great for me. If I come across a post where someone tries to get "fancy" with a hard to read color, I just move on. Ain't gonna be a life changing event if I don't read it.