Currently, when neovim is run in the embedded terminal with the default
colorscheme, it cannot detect the background color because Vaxis does
not support the corresponding OSC sequences. This leads to neovim
defaulting to dark
and this has to be set manually if the background
is in fact light.
This is relevant for instance for aerc, which itself has a light background compliant colorscheme, but the editor opened when composing a message cannot adapt automatically.
Incidentally, neovim's built-in terminal did not support this either until quite recently. Here is the relevant pull request adding the support.
Karel Balej referenced this ticket in commit ad972cc.