You could try "reading mode" which many browsers have.  That would help out the contrast and background colours, and you don't have to mess with addons.

Wow, Dormouse, thanks for the analysis. 
So far, I have not been pleased enough with any of the markdown editors I've tried so far.  Whilst I really appreciate everything saved in text and therefore fetchable from other programs, I don't like switching between editing directly and in markdown.  I just don't care about markdown, I guess.  I'm pretty happy with TreeDBNotes, although it needs some improvements and needs some promise for the future, but it's still working well for me.  I'll keep an eye out for anything new and interesting, though.