Update: Based on the discussion here and in other places I added the following (well, technically I did something different in my colorscheme, but in the end it translates to that)
vim.api.nvim_set_hl(0, 'Normal', {})
This reverts the weird text and background colors to the previous behavior of ... not setting them.
With update 0.10 Neovim behavior changed regarding text color and background color.
I use a color theme that does not set those and previously this worked perfectly fine. Neovim simply used the font color defined in the terminal and had a transparent background.
Now the background is #14161b and the font color is #e0e2ea. Neither of the colors is configured ANYWHERE in my whole setup. Neither in the colorscheme, nor in my terminal configuration, nor in my Neovim configuration.
Is there a sane way to revert this to the old behavior? (i.e. use the font color configured in the terminal’s configuration and use transparent background.)