The light theme doesn't look anywhere near as good IMO. It's too visually noisy - the text isn't as clear and the bright colors compete for attention with it.

Having a lightmode as an option is good, but if I may be blunt this ain't it and especially if this is going to be people's first impressions of the site.

I'm under the impression that darkmode is the more widely accessible option; whitemode is the common standard only due to cultural inertia. Is this wrong?

I'd personally be inclined to take darkmode as the default, due in part to the above and in part to personal preference.