Add a light mode to libcu++ website and make it more friendly to color vision deficiency

A light mode would be nice.

I wonder if the libcu++ site is friendly to people with color vision deficiency? (I'm always wary of NVIDIA green, especially on top of black or dark grey, for this reason.) Even I find the green on that site a bit hard to read, and I don't think I have CVD.

This link shows the site filtered to demonstrate various forms of CVD. https://www.toptal.com/designers/colorfilter?orig_uri=https://nvidia.github.io/libcudacxx/api.html&process_type=grey