This would seem to me a contrast issue. It could be fixed by having a “high-contrast” setting which would make the UI “bold black on white” instead of “light grey on white”. That said, I don’t think there are really places in Everdo where important text is in light grey (please do point out if you see any). The titles of items are almost black and the descriptions are a bit lighter. The font sizes are smaller in some places, so some labels may look lighter than they really are.
Themes are a different and much more complicated feature. It’s also much less critical compared to some other suggested features.
In any case, this topic is exactly for finding and fixing contrast issues, but implementing support for themes is not the way to fix it, at least not now.