Really uncomfortable reading here (regular PC in a regular office environment) and according to Chrome's inspector it's #777. If I remove the "proxima nova" font it improves a bit because of Helvetica's added weight.
Really unfortunate design for an article about design, and blaming the users' browsers, operating systems, lighting conditions or etc. is NOT the correct approach.
On a more useful note, I enjoyed Fred Brooks' "The Design of Design", on the process of design rather than on visual design.
If you have trouble reading that on a white background, then the problem is with your screen, not with the page.