I prefer the default theme on Windows 10 as there the title bars of the other windows like Settings and Explorer use the same style, which is the color defined in 'Colors' in the settings. Using the 'Light' theme makes Firefox looking different which isn't nice.

Haven't seen any other black title bar on my Mac though...

I think light is the default for the release version, but devs seem to like dark UIs for some reason.

There's a Light theme, a Normal theme and a Dark theme.

Dark is the default for the Developer Edition, which is what you're thinking of. But Normal is the default for Stable. Light is almost the same as Normal, but with a few more elements in white.

The reason is eye strain in poorly lit offices :)