If we look for a long time on a web page - which is natural for a live chess broadcast - then dark background colors are better for the eyes. The contrast at TCEC is ok even if I turn down the screen's brightness (which I usually do for the same reason).
Good web designers are aware of that, and at least offer an alternative "dark" design. See for example the ying/yang symbol at the top, at
http://en.lichess.org/ TCEC being dark by default, is perfect for me. But if the demand for a "light" design is big, maybe such an alternative setting becomes considerable. For me it would feel like sitting in front of a flood light, though.