Insufficient text contrast has been noted in reviews of Plasma, for example http://www.ocsmag.com/2017/10/18/plasma-5-11-keep-the-momentum-going/
The new color is just a darker gray, not solid black, so there is still never a solid-black-on-pure-white contrast situation that can be too extreme for some.
Compare old vs new and look at the text and tooltips:
Before:
{F5456910}
After:
{F5456911}
Before:
{F5456920}
After:
{F5456922}
IMHO the second ones are clearer and less muddy, and therefore **easier** on the eyes, not harder.